Second y-axis

[singerb]

I appreciate the general simplicity of design with this library, rather than trying to be a kitchen sink. However, given that you already support different chart styles on the same chart (i.e. bars and lines), it would be great to allow for 2 y-axes, so that you can make a classic bar chart showing % rate of change as a line on a single graph. Currently this only works if the first dataset is roughly the same order of magnitude as the rate of change.