add support for power scales with custom exponent

[davelandry]

Originally posted by @xscotta in this issue

I have used the following options to tease out more of the data when the difference between max/min is great.
.yConfig({ scale: "sqrt" }) — This applies a fixed exponent of 0.5 to the axis
.yConfig({ scale: "log" }) — This applies a natural log(e) to the axis

If I want to specify an exponent of 0.4 or 0.3, can I use "pow"?
.yConfig({ scale: "pow" }) — And then specify the exponent somewhere?

The documentation on D3plus http://d3plus.org/examples/d3plus-axis/scale-time/ says All of the scales made available by d3-scale are usable for an Axis by passing the name of the scale, in all lowercase, to the .scale( ) method.

Thanks for your help, I looked through the D3plus code and documentation but couldn't work it out.