domainMid of diverging color scheme ignored when it falls outside data range

(See also: https://github.com/vega/vega-lite/issues/6544)

I’ve made a chart that uses a diverging color scheme with "domainMid": 0 with the expectation that this would render negative numbers with the lower half of the color scheme and positive ones with the upper half. But when the data contains only positive numbers, Vega warns in the console that domainMid falls outside of the domain:

WARN Scale domainMid exceeds domain min or max. 0

…and proceeds to map positive numbers to the whole domain, and in a very coarse fashion that only seems to use the extreme colors from the range, as illustrated by this example (which I’ve tried to simplify as much as I could):

• Reproduction

When I add a -1 value to the data set, it renders just perfect:

• Same spec, but data has a negative one

When I filed a ticket on this against Vega-Lite, @domoritz offered a workaround on Vega-Lite that compiles to a Vega spec similar to this one, with a domain to which [0] has been added:

• Domain union with zero

[domoritz]

Thank you for filing the issue.

@jheer I think the problem is best fixed in Vega (if at all) so the behavior is consistent across tools but if you disagree, we can see how we can address it in Vega-Lite instead.

[sacundim]

Thanks guys!