d3.interpolateString(a, b)
Returns an interpolator between the two strings a and b. The string interpolator finds numbers embedded in a and b, where each number is of the form understood by JavaScript. A few examples of numbers that will be detected within a string: -1, 42, 3.14159, and 6.0221413e+23.
For each number embedded in b, the interpolator will attempt to find a corresponding number in a. If a corresponding number is found, a numeric interpolator is created using interpolateNumber. T