ChartMoE: Mixture of Diversely Aligned Expert Connector for Chart Understanding

Thanks for your detail review!

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Q1: About Novelty and Tradd-off of MoE

MoE on multimodal language models has been explored in other domains. There's a trade-off between the performance gain and the increase of model parameters and inference time.

We would sufficiently highlight the distinctions between ChartMoE and other MoE models in MLLMs. Some prior work, such as MoE-LLaVA[1], DeepSeek-VL[2], and CuMo[3], has employed MoE architectures in MLLMs. However, these approaches all apply MoE to LLMs or ViTs to increase model capacity, introducing a large number of learnable parameters to boost performance. In contrast, our ChartMoE introduces several distinctive innovations:

1. Motivation: Our goal is not to expand model capacity but to enhance the model's chart comprehension through alignment tasks while preserving performance on other general tasks. Hence, we retain the original connector parameters as one expert initialization manner.

2. Initialization: Unlike previous methods that rely on random or co-upcycle initialization, we leverage diverse alignment tasks for expert (connector) initialization. This approach enables ChartMoE to exhibit remarkable interpretability (Fig.6,11,12). From the perspective of visual tokens, experts aligned with Table and JSON focus more on regions with text, such as titles and legends, excelling in tasks similar to OCR. In contrast, the Code expert focuses more on data points and trend directions, excelling in visual logical reasoning and overall analysis.

3. Complexity: We are the first to apply MoE exclusively to the MLP connector (projector) in LLaVA-like MLLMs. In ChartMoE (based on InternlmXC-v2), the MoE architecture introduces minimal additional parameters (model size 8.364B $\rightarrow$ 8.427B, + 63M$\uparrow$ only) and training complexity (Fig.4). It also shows negligible impact on inference speed (0.945 $\rightarrow$ 0.952 seconds per QA on ChartQA test set) and peak memory usage (23.72 GB $\rightarrow$ 23.86 GB, fp16 on A100-40G GPU).

We have added this discussion to Appendix D.1 to make our paper more comprehensive. We hope our explanation helps you reassess the novelty of ChartMoE!

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Q2: More Theoretical Analysis of MoE

Our motivation for using the MoE connector is primarily based on the following two points:

1. Different structured texts contain varying core information and information volumes, leading the aligned experts to focus on different regions of the charts.

2. We aim to enhance chart understanding capabilities while preserving the original model's performance on general tasks.

The visualization results (Figures 6, 11, 12) show that the vanilla expert tends to handle background tokens, the table&Json experts focus more on text and textures, and the code expert pays more attention to data trends. This insight led us to remove the bz-loss from the standard MoE architecture, as the visual tokens are not evenly distributed, and forcing an equal workload for each expert is not optimal. We will explore more in-depth theoretical analysis in our future work.

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Q3: More Ablation Study

Have you ever tried training on your alignment data with random initialization of expert parameters and balanced loss from scratch? It will better prove the significance of this work.

Thank you for providing an important baseline setting in the ablation study! We have added the results for this setting in Table 5. Align refers to using ChartAlign for alignment, while Init. refers to random initialization without alignment. The experimental results demonstrate the significance of ChartMoE.

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We are glad to see your recognition of our contribution. We would be happy to answer any remaining questions or concerns!

Reference:

[1] Moe-llava: Mixture of Experts for Large Vision-Language Models, Arxiv 2024.

[2] Deepseek-vl: Towards Real-world Vision-Language Understanding, Arxiv 2024.

[3] CuMo: Scaling Multimodal LLM with Co-Upcycled Mixture-of-Experts, Arxiv 2024.

Dear Reviewer Ka6r,

With the discussion phase ending in 3 days, we would greatly appreciate any further guidance or confirmation from your side. We wanted to follow up on our previous response to your valuable suggestions. Your insights are very important to us, and we are eager to ensure that we have addressed all concerns appropriately.

Warm Regards, :-)

Thank you for your review and detailed comments! Your suggestion is valuable for us to make this paper comprehensive.

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Q1: ChartMoE Base on Other MLLMs

Thank you for your very reasonable suggestions. ChartMoE can be applied to all LLaVA-like MLLMs. Therefore, we conducted experiments based on LLaVA-v1.5-7B, using 10% alignment data (Tab. 1) and ChartQA training data, and the results are shown in the table below.

As shown in the table, our proposals significantly improve the base model. This is partly because LLaVA is trained with fewer chart data, leading to a lower baseline performance, and also indicates that the additional alignment data and MoE connector greatly enhance chart understanding. We have added this discussion to Appendix D.2 to make our paper more comprehensive.

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Q2: About Limitations of ChartMoE

ChartMoE has two limitations:

1. Dependency on alignment tasks. ChartMoE requires chart-Table/JSON/Code alignment tasks for initialization. Non-chart multimodal tasks need new alignment designs to initialize MoE experts.

2. Limited flexibility. Modifying the projector into a multi-expert architecture makes ChartMoE non-plug-and-play like LoRA. We are required to retrain the router network when new experts are coming.

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Q3: About Metric on ChartQA

Are other comparison models also limited by ChartQA evaluation metric？

Yes, other methods are also affected by the evaluation criteria. We report results that we have reproduced ourselves or re-evaluated using new criteria based on the inference result files provided by the original authors.

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Q4: About ChartMoE-Align Details

We have updated Appendix C, where we detail the construction process of ChartMoE-Align. We provide code templates for chart type-agnostic attrbutions and adopt ruled-based methods to fit different chart types. Please refer to Appendix C for details. We will release all sources of ChartMoE-Align and ChartMoE for research purposes.

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We hope to have addressed your concerns and are thankful for your recognition of our contributions!

Dear Reviewer WXLS,

With the discussion phase ending in 3 days, we would greatly appreciate any further guidance or confirmation from your side. We wanted to follow up on our previous response to your valuable suggestions. Your insights are very important to us, and we are eager to ensure that we have addressed all concerns appropriately.

Warm Regards, :-)

Q3: Synthetic Data Might Limit the Model’s Ability

Also, the training data, being mostly synthetic, might limit the model’s ability.

Yes, real data is typically more effective than synthetic data. However, collecting a sufficient number of real-world charts with meta tables is challenging, and obtaining charts with meta JSON and code is even more difficult. Recent studies [4-5] have shown that synthetic data can significantly aid model training. The synthetic data in ChartMoE-Align has also been carefully diversified and quality-controlled.

Additionally, we only use synthetic data for the alignment stages. During the SFT stage for training the MoE router and LLM, we still use real data from MMC, ChartQA, and ChartGemma, which helps ChartMoE achieve SOTA performance on multiple benchmarks. The ablation experiments in Tables 5-8 also demonstrate the effectiveness of synthetic data in ChartMoE training.

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We hope to have addressed your concerns.

Reference:

[1] Moe-llava: Mixture of Experts for Large Vision-Language Models, Arxiv 2024.

[2] Deepseek-vl: Towards Real-world Vision-Language Understanding, Arxiv 2024.

[3] CuMo: Scaling Multimodal LLM with Co-Upcycled Mixture-of-Experts, Arxiv 2024.

[4] Self-Rewarding Language Models, Arxiv 2024.

[5] Reinforced Self-Training for Language Modeling, Arxiv 2024.

Thanks for your review and detailed comments! We hope the following discussion can address your concerns!

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Q1: About the Novelty of MoE in MLLMs

The use of MoE in MLLMs is not particularly novel, as several prior works have already explored MoE structures to enhance model performance. From an innovation standpoint, this reliance on MoE does not introduce a distinctly new approach and could be considered a weakness in terms of contribution.

We would sufficiently highlight the distinctions between ChartMoE and other MoE MLLMs. Some prior work, such as MoE-LLaVA[1], DeepSeek-VL[2], and CuMo[3], has employed MoE architectures in MLLMs. However, these approaches all apply MoE to LLMs or ViTs to increase model capacity, introducing a large number of learnable parameters to boost performance. In contrast, our ChartMoE introduces several distinctive innovations:

1. Motivation: Our goal is not to expand model capacity but to enhance the model's chart comprehension through alignment tasks while preserving performance on other general tasks. Hence, we retain the original connector parameters as one expert initialization manner.

2. Initialization: Unlike previous methods that rely on random or co-upcycle initialization, we leverage diverse alignment tasks for expert (connector) initialization. This approach enables ChartMoE to exhibit remarkable interpretability (Fig.6,11,12). From a visual token perspective, experts aligned with Table and JSON prioritize text regions like titles and legends, excelling in OCR-like tasks. In contrast, the Code expert emphasizes data points and trend directions, demonstrating strengths in visual logical reasoning and overall analysis.

3. Complexity: We are the first to apply MoE exclusively to the MLP connector (projector) in LLaVA-like MLLMs. In ChartMoE (based on InternlmXC-v2), the MoE architecture introduces minimal additional parameters (model size 8.364B $\rightarrow$ 8.427B, + 63M$\uparrow$ only) and training complexity (Fig.4). It also shows negligible impact on inference speed (0.945 $\rightarrow$ 0.952 seconds per QA on ChartQA test set) and peak memory usage (23.72 GB $\rightarrow$ 23.86 GB, fp16 on A100-40G GPU).

We have added this discussion to Appendix D.1 to make our paper more comprehensive. We hope our explanation helps you re-assess the novelty of ChartMoE. Thank you!

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Q2: MoE Architecture and Differentiated Alignment Tasks Add Complexity

The multi-expert structure, along with the diverse alignment tasks, adds significant complexity to ChartMoE’s architecture.

1. Model Complexity. In Q1, we introduced the minimal increase in model parameters, training difficulty, inference speed, and inference memory requirements introduced by the MoE connector (projector). This is because we only replace the linear projector with an MoE at the connector position, making the overall changes minimal.

2. Training Complexity. ChartMoE does not significantly increase computation costs. The expert alignment process in ChartMoE uses 800k training data (500k tables for expert 1 / 200k JSONs for expert 2 / 100k codes for expert 3), and the training parameters include only a single MLP expert. The computation cost of the alignment process is equivalent to training an MLP Connector with part of the ChartMoE-Align data (800k), where the difference is the training data is split by task type for 3 stages.

Therefore, ChartMoE introduces almost no additional model complexity or training complexity compared to other MLLMs with LLaVA structures. We have added this discussion to Appendix D.1 to make our paper more comprehensive. We hope our explanation is helpful for you.

Dear Reviewer 6BC6,

With the discussion phase ending in 3 days, we would greatly appreciate any further guidance or confirmation from your side. We wanted to follow up on our previous response to your valuable suggestions. Your insights are very important to us, and we are eager to ensure that we have addressed all concerns appropriately.

Warm Regards, :-)

Q3: Modality-Specific Contributions

It would be beneficial for the paper to elaborate on the unique contributions of different representations (JSON, code, and chart) in the context of chart-related tasks. Understanding how each representation impacts the model's learning and performance could provide insights into optimizing future models for similar tasks.

Thanks for your suggestions. To demonstrate the individual contributions of each component, we conduct two ablation studies:

1. Table 7 shows the performance of ChartMoE when experts are manually activated (without considering the router). The vanilla expert demonstrates the best performance, which aligns with its activation frequency and the number of tokens it processes. The code expert also performs well, indicating that this modality brings significant benefits to the system.

2. Table 8 presents the results when using a single linear projector instead of the MoE architecture, trained with different alignment tasks. Without supervised fine-tuning (SFT) on ChartQA, each alignment task degrades the performance, likely due to the significant differences between alignment tasks and QA tasks. After SFT on ChartQA, all three experts outperform the vanilla expert, demonstrating the effectiveness of the alignment tasks.

We illustrate each expert's preference for selecting visual tokens in Figures 6, 11, and 12. From the perspective of visual tokens, experts aligned with Table and JSON focus more on regions with text, such as titles and legends, excelling in tasks similar to OCR. In contrast, the Code expert focuses more on data points and trend directions, excelling in visual logical reasoning and overall analysis.

We hope these experimental results are helpful to you.

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Q4: Necessity and Efficiency of Large-Scale Alignment Dataset

Can a smaller but more carefully designed dataset achieve similar alignment effects?

We believe it can. However:

1. Collecting real, finely annotated Table-Chart pairs requires more resources, and collecting Table-JSON-Code-Chart quadruples is even more challenging.

2. The alignment task is inherently a relatively simple and fixed task. Therefore, we believe that using the cleaned and quality-checked ChartMoE-Align dataset is sufficient. This is reflected in the data ratios shown in Table 1, where the loss for certain modalities saturates after training for a specific number of iterations. As a result, we do not use the full amount of data for each modality.

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We hope to have addressed your concerns!

Reference:

[1] OneChart: Purify the Chart Structural Extraction via One Auxiliary Token, Arxiv 2024.

[2] ChartReformer: Natural Language-Driven Chart Image Editing, Arxiv 2024.

Thank you for your review and detailed comments! We are glad to see your recognition of our contribution. You can find below our detailed response to your questions and concerns. Please let us know if you have any further concerns or suggestions.

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Q1: Details on ChartMoE-Align Construction

Thanks for your suggestions! We have updated Appendix C, where we detail the construction process of ChartMoE-Align. Specifically, ChartMoE-Align uses real-world tables (from ChartQA and PlotQA) and GPT-generated tables (from ChartY provided by OneChart[1]). We further filter the data in ChartY, removing duplicates and maintaining a balance of chart categories. We adopt the JSON template provided by ChartReformer[2] and further categorize attributes into chart type-specific and type-agnostic. During code generation, we use fixed templates for type-agnostic parts and rule-based methods to ensure faithful representation of each attribute for type-specific parts. Finally, we discard all quadruples that produce rendering errors or warnings. To ensure data quality, we randomly sample 200 quadruples and evaluate them using both human and GPT4 judges to assess chart quality and the match between chart, table, JSON, and code. The evaluation results show that ChartMoE-Align is sufficient for the alignment training task. Please refer to the appendix for a better reading experience. Combined with the description in Section 3.2, we hope this addresses your concerns.

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Q2: Clarification of Experiment Results

The paper briefly mentions that the proposed method shows weaknesses in some settings compared to baselines, as detailed in Tables 2 and 3. However, these points are not adequately addressed or explained. A more thorough analysis of why ChartMoE underperforms in these instances would be valuable for readers and for future improvements to the method.

Thank you for your suggestions! We add a discussion in Appendix D.3 regarding why ChartMoE performs less well than previous SOTA in certain settings. In Tab.1, our ChartMoE significantly outperforms SOTA. However, some models perform better than ours on the Augment part of the ChartQA test set. Given that the Augment part of ChartQA is considerably easier than the Human part, we conduct a more detailed analysis. We analyze the performance of various models on numeric (Human: 43%, Augment: 39%) and non-numeric (Human: 57%, Augment: 61%) questions in ChartQA. As shown in the following table, ChartMoE excels in all subcategories except for non-numeric questions in the Augment part. We find that ChartMoE's errors primarily occur in string-matching tasks. For instance, the prediction of It is between 2003 and 2005 is marked incorrect if the ground truth is (2003, 2005). High accuracy in this category may indicate overfitting instead.

Table 3 shows the results on ChartBench, where ChartMoE does not achieve the best performance in the Pie and Box subcategories. This is because:

1. The sample sizes for these two subcategories are small, which may introduce random bias.

2. A more detailed error analysis reveals that most of the errors occur in the Chart Type tasks for these subcategories (e.g., <u>Q: This is a combination chart, not a pie chart.</u>). We further instruct ChartMoE to provide reasons for its judgments (<u>A: This graph contains multiple charts, including pie, so it can be considered either a combination chart or a pie chart, depending on what you are asking about.</u>). Therefore, the performance gap is due to the ambiguity in these cases combined with the small sample size of these subcategories, making it non-representative.

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Dear Reviewer jNMd,

With the discussion phase ending in 3 days, we would greatly appreciate any further guidance or confirmation from your side. We wanted to follow up on our previous response to your valuable suggestions. Your insights are very important to us, and we are eager to ensure that we have addressed all concerns appropriately.

Warm Regards, :-)