scaleDiverging for domain [1,1,0] produces wrong results

[MaPePeR]

The d3.scaleDiverging Method returns wrong results if the domain is reverted and the left domain border is equal to the midpoint.

const s = d3.scaleDiverging().domain([1, 1, 0]);
for (const i of [-0.5, 0, 0.25, 0.5, 0.75, 1, 1.5]) {
    console.log(i, s(i));
}
/* prints
-0.5     0.5
 0       0.5
 0.25    0.5
 0.5     0.5
 0.75    0.5
 1       0.5
 1.5     0.25
*/

Having the domain sorted and applying 1-x to the scale produces the expected results:

const s = d3.scaleDiverging().domain([0, 1, 1]);
for (const i of [-0.5, 0, 0.25, 0.5, 0.75, 1, 1.5]) {
    console.log(i, 1 - s(i));
}
/* prints
-0.5     1.25
 0       1
 0.25    0.875
 0.5     0.75
 0.75    0.625
 1       0.5
 1.5     0.5
*/

I encountered this because I wanted to use the RdBu colormap and map values [0,1] to [0.5,1] (blue part) and [1,max(data)] to [0, 0.5] (red part):

const colorMap = d3.scaleDiverging().domain([Math.max(maxValue, 1), 1, 0]).interpolator(d3.interpolateRdBu);

and had to change it to this:

const colorMap = d3.scaleDiverging().domain([0, 1, Math.max(maxValue, 1)]).interpolator(x => d3.interpolateRdBu(1 - x));

I use https://cdn.jsdelivr.net/npm/[email protected]/dist/d3.min.js.

[mbostock]

Right, diverging scales don’t currently support reversed (or “descending”) domains. We’d need to add that feature.