AbstractWe present MatplotAlt, an open‐source Python package for easily adding alternative text to Matplotlib figures. MatplotAlt equips Jupyter notebook authors to automatically generate and surface chart descriptions with a single line of code or command, and supports a range of options that allow users to customize the generation and display of captions based on their preferences and accessibility needs. Our evaluation indicates that MatplotAlt's heuristic and LLM‐based methods to generate alt text can create accurate long‐form descriptions of both simple univariate and complex Matplotlib figures. We find that state‐of‐the‐art LLMs still struggle with factual errors when describing charts, and improve the accuracy of our descriptions by prompting GPT4‐turbo with heuristic‐based alt text or data tables parsed from the Matplotlib figure.