I believe you can treat it as if it were an image by putting it an element with attributes role="img" aria-label="description of the diagram goes here"
The problem is, having to describe a diagram using text defeats the purpose of the diagram. Must better is to use diagrams that render as SVG, so screen readers can read them. [0]
A flowchart involving labeled boxes with arrows between them is going to be more accessible as an SVG than a PNG, sure, but you're still going to need to explain what the arrows are doing, otherwise a screenreader user will be quite lost.
