function d3_interpolateHcl(a, b) {
    a = d3.hcl(a);
    b = d3.hcl(b);
    var ah = a.h, ac = a.c, al = a.l, bh = b.h - ah, bc = b.c - ac, bl = b.l - al;
    if (isNaN(bc)) bc = 0, ac = isNaN(ac) ? b.c : ac;