Max fillOpacity is replaced with 1

[nobarany]

When fillOpacity is a function or property, the maximum value is replaced with 1. For example:

...
// fillOpacity: 0.2 works fine
fillOpacity: d => 0.2
...

results in fill-opacity="1"

[mbostock]

Yep, that’s because when you specify the fill opacity as a function, you are supplying abstract rather than literal values. For example, 0.2 might represent kilometers or degrees Kelvin. If you want Plot to interpret the fill opacity literally, you need to set the type of the opacity scale to identity:

Plot.plot({
  opacity: {type: "identity"},
  marks: […]
})

Alternatively, as of Plot 0.6.4 (which is released to npm but FYI is not yet the default on Observable), you can opt-out on a specific channel:

Plot.dot(data, {…, fillOpacity: {value: (d) => 0.2, scale: null}}

We could perhaps add some value inference if all values for a channel bound to the opacity scale are in the range [0, 1]; we do something similar for channels bound to the color and symbol scale. I’ll file a separate feature request for that.

[nobarany]

Oh, great, thanks! I'm using an explicit scale. scale:null fixed it.