Use midpoint of range with collapsed domains?

[mbostock]

Extracted from #113: when a scale has a collapsed domain (or domain segment), we currently map the value to the start of the corresponding range (or range segment). For example:

var x = d3.scaleLinear().domain([4, 4]).range([0, 10]);

x(4); // 0

This is of course arbitrary; it should be valid to return any value in the corresponding range. The current implementation uses constant(0) in this case, but it should be a matter of using constant(0.5) instead:

d3-scale/src/continuous.js

Lines 9 to 13 in 4c8c282

	export function deinterpolateLinear(a, b) {
	return (b -= (a = +a))
	? function(x) { return (x - a) / b; }
	: constant(b);
	}

Though, we’ll now have to check explicitly for isNaN(b).

[mbostock]

Fixed in #156.