d3.interpolate(a, b)
Returns an interpolator between the two arbitrary values a and b. The interpolator implementation is based on the type of the end value b, using the following algorithm:
If b is null, undefined or a boolean, use the constant b. If b is a number, use interpolateNumber. If b is a color or a string coercible to a color, use interpolateRgb. If b is a date, use interpolateDate. If b is a string, use interpolateString. If b is an array, use interpolateArray. If b is coercible to