I think we’re in agreement, just that you understood what I wrote a bit differently from how I intended. By “described in the visible text” I didn’t mean “expounded at length”. I’m talking most commonly about the sort of captions newspapers printed with images, typically 5–30 words; but also about things in the text that refer to the figure, like “here are the benchmark results:” and “we see that X is 57% faster than Y”.

There’s no good way of exposing lengthy information about a chart to screen readers. Don’t try doing it via alt, it’s a bad experience; and don’t try doing it by screen-reader-only text, that just gets confusing. The longdesc attribute was an attempt to do this, but it failed to gain traction among users or assistance tech. Consensus now is that you should instead include in the caption a link to a page that focuses just on the chart, which can then include more detail for the screen readers.

I think ideally those charts would be SVGs that can have elements with their own ARIA labels and be navigated by a screen reader. It would be very clumsy to describe an entire chart in an alt text string.