plotter: Image and ColorBar don't adapt to non-linear axes

Created by: ctessum

What are you trying to do?

I would like to plot an Image or Color bar on a plot with a non-linear axis.

What did you do?

I will submit a pull request with a reproduce

What did you expect to happen?

The image should be warped to account for the non-linear axis.

What actually happened?

The image assume the axis was linear even when it wasn't

What version of Go and Gonum are you using?

go 1.10, gonum master

Does this issue reproduce with the current master?

yes