I just noticed this feature when I applied Comfort Update today.

I clicked it (after generating HTML report), and I downloaded the ZIP file with lots of png files -- but all of them are black (no graphs).

Any idea why this might happen?

What browser are you using?
The statistics rely on canvas elements, so it may be that the image generation is quirking in you browser.

I examined more of the exported images, some are black, some have graphs in them. I'm using Google Chrome on Windows 10. When I look up question ID for black images, I don't see any reason why it shouldn't have a graph shown, as the question does in the Statistics Report.

As an aside, it would be nice if on the Statistics Report, we could include Question ID for each question, so we could easily associate the image back to the image in the report (instead of having to look up questionID in the List Questions screen).