Legend not showing when values are `var(...)`

[vincent-octo]

Hello,

When using color: {legend: true} and values are "var(--something)" then the legend does not appear.
Additionally, I could not see any warning or error message that could help in understanding why the legend doesn't show.

Here is an notebook that reproduces the bug: https://observablehq.com/d/fbd79535abd8788a
Changing fill: "test" with fill: "sex" will display the legend again, so I am assuming that the "var(...)" content of test is to blame here.