Does not support polylinear color scale

[amergin]

Note that this implementation does not support polylinear color scale. For instance,

var linear = d3.scale.linear()
.domain([-0.55, -0.10, 0, 0.16, 1])
.range(['blue', "#b3b3ff", 'white', "#ffb3b3", 'red']);

is a valid linear color scale which is not properly drawn using this library. This kind of scale can be drawn using this implementation.