The opacity and fillOpacity channels do not faithfully render the opacity value in the column

[quantoid]

When using the fillOpacity channel (via Altair), a value of 0.0 in the column actually gets rendered at around 0.38 opacity. Perhaps this is so that marks don't disappear completely, but it seems to be undocumented and it's not obvious how to make opacity exactly match the column values.

In the end I had to specify a domain starting from 0.3:

"fillOpacity": {"field": "opacity", "type": "quantitative", "scale": {"domain": [0.4, 1.0]}}

Now an opacity of 0.0 will disappear (if that's what you want).