In http://www.w3.org/html/wg/drafts/html/master/embedded-content-0.html#graphical-representations:-charts,-diagrams,-graphs,-maps,-illustrations there are examples of different kinds of images. The recommended alt text for pie charts clearly state "Pie chart" but none of the other image types state their type.

The results of the first and second WebAIM screen reader user studies (2008, 2009) showed a preference among survey takers for alt text to mention what kind of graphic it refers to [1] [2], possibly because the alt text in those particular questions already lack context. That's not to say everyone always wants graphics labeled, but that context of a graphic likely matters to many relying on alt text.

This makes sense for context reasons: say students are discussing a page in an online textbook containing 2 bar graphs and 1 pie chart, all showing different aspects of the same subject. With the context of what kind of graphic available, the students can more easily refer to the graphics by type, or search by type. Not only is the information in the graphic conveyed visually, but also what kind of graphic it is as well.

As the WebAIM studies caution, this doesn't mean every graphic should be labeled this way, but on a page containing multiple complex graphics I feel it should be recommended.


[1] http://webaim.org/projects/screenreadersurvey/#images
[2] http://webaim.org/projects/screenreadersurvey2/#images