When describing graphs and charts and other complex images, it is important to consider the context of the problem and the question being asked.
Your next question(s) should be: What is the context? What question is being asked about the graph? If my question was: What is the equation for the line shown below? A poor alt text would be: “Graph of the equation y = (1/2)x + 1.” This gives away the answer! Or course, it depends not only on the questions being asked, but also the context. If you have just been demonstrating how to write the equation of a line when given two points, your alt text could give a few points (e.g. “A linear graph containing the points (-2,0), (0,1), (2,2), (4,3) and (6,4)). So you need to be concise but also give enough information that students will be able to address your question. Also, the type of language you use might vary according to the math level (e.g. a student in a calculus class vs a student in a developmental-level class).
Here are some good guidelines that specifically address mathematics content.
Powerpoint used in the workshop