function d3_interpolateHsl(a, b) {
    a = d3.hsl(a);
    b = d3.hsl(b);
    var ah = a.h, as = a.s, al = a.l, bh = b.h - ah, bs = b.s - as, bl = b.l - al;
    if (isNaN(bs)) bs = 0, as = isNaN(as) ? b.s : as;