Describing the information conveyed within complex images such as graphs, charts, and diagrams can be challenging. Images must have text alternatives in order to be accessible to people who are unable to see the images, such as people who are blind and using screen reader software to read a document. Text alternatives are designed to describe the information or function the images are intended to convey. Fortunately, there are a few excellent resources that provide guidelines for describing complex images, supplemented with a variety of examples.
WGBH's Carl and Ruth Shapiro Family National Center for Accessible Media has published Effective Practices for Description of Science Content within Digital Talking Books. This publication is the result of a 4-year effort that included multiple surveys with describers and with students and scientists with vision loss and research of preferred practices for description of visual information in textbooks and journals. NCAM undertook this research with support from the National Science Foundation (NSF, grant # 04535663). This publication includes a set of guidelines, plus examples for describing bar charts, line graphs, Venn diagrams, scatter plots, tables, pie charts, flow charts, standard or complex diagrams, and illustrations. Although written specifically for digital talking books, the guidelines and examples provided in this publication are broadly applicable to all complex images, including those distributed via web pages.
NCAM also contributed to Image Description Guidelines, published by DIAGRAM Center (Digital Image And Graphic Resources for Accessible Materials) at Benetech. These guidelines are organized into two sections, General Image Description Guidelines (covering general style, language, formatting, and layout) and Specific Image Description Guidelines. The second section includes guidelines and examples for describing specific categories of images, including art, photos, and cartoons; chemistry; illustrated and relational diagrams; graphs; maps; mathematical images; tables; and images of text.
Finally, the World Wide Web Consortium provides guidelines and examples for describing complex images within their Web Accessibility Tutorial on Images.