a scale's "mid" needs an odd number of colours (a middle stop to pin); {n} were given, so "mid" was ignored.

Warnings reference › formatting.scale.mid.even

Identifier

formatting.scale.mid.even

Raised by

The grid (core)

Since

1.68.2

What happened

mid pins the middle colour stop of a colour scale to a particular value, and with an even number of colours there is no middle stop to pin. The setting is ignored and the stops are spread evenly, so a diverging scale will not be centred where you asked.

The fix

Give the scale an odd number of colours - three is the usual diverging scale - or drop mid.

The smallest fix

rules: [{ scale: { colours: ['#c00', '#eee', '#080'], mid: 0 } }]

Reference

Covered in full at the API reference.