I guess you’d need to distinguish diagrams and free form drawing.
Charts from tabular data
Classic bar, line, or pie charts are generated from tabular data. As long as the tool allows for navigation and creation of tables with a screen reader, a screen reader user can structure these data points.
Diagrams and graphs from text
Graphs and diagrams visually present semantically linked data. A good diagramming tool doesn’t allow for free form, but already uses standard symbol libraries and can check for loops or missing connections.
That data can also be linked with a screen reader by writing in markup languages. mermaid is such an example. The graph then is usually being laid out by an algorithm.
would produce the following rendering
Try it in the Mermaid Live Editor
Mermaid allows for
- Sequence Diagram
- Class Diagram
- State Diagram
- Entity Relationship Diagram
- User Journey
- Pie Chart
- Quadrant Chart
- Requirement Diagram
- Gitgraph (Git) Diagram
SVG is another, very powerful markup language for any kind of vector drawing, which can simply be typed.
Yet another example is LibreOffice, which allows you to create mathematical formulae by means of point and click, or by typing in MathML syntax.
Syntax can be checked and errors recognised.
When I was looking for an accessible Content Management System back in the days, the certified variant also used a markup language (long before markdown). So I‘m assuming it to be an accepted solution for assistive technology. I‘m not an AT user myself and have no insights into how practical that actually is.