Per the small text at the bottom of the poster I would say they are based on shipment volumes -so how much is available. The lighter colour probably indicates variation depending on how good the growing season is

Yes, this is correct. Realistically, you only care "is it there", and "is it there and good", and those are the two levels. For SF, I'm using shipment volume as a proxy for ripeness, on the assumption that the best produce occurs when you're shipping the most of it. The shaded parts are when the annual history says that it's only good in some years but not others.

I decided against a legend because I thought that it was self evident. Maybe not... https://vertumnus.fyi/about/transcript?view=topic%3Aposter-f...