i made a page for this:
page: https://jumear.github.io/stirfry/iNat_calendar_heatmap
code: https://github.com/jumear/stirfry/blob/master/iNat_calendar_heatmap.html
as an example, this would visualize American Robin observations in Texas between 2019-01-01 and 2025-12-31: https://jumear.github.io/stirfry/iNat_calendar_heatmap?taxon_id=12727&place_id=18&d1=2019-01-01&d2=2025-12-31
it might be useful for a use case like this, too: https://forum.inaturalist.org/t/enhance-observation-calendar-contrast-differentiate-shades-for-high-volume-days/54271.