Custom scale transformations

I'd like to be able to make a custom scale. My particular use-case is transforming unit magnitudes, for example, by having a scale that multiplies values by 1e6.

In plotnine, custom transformations are supported via arbitrary `lambda`s / python expressions, but I realize that's tricky given your backend. Maybe an affine transformation (similar to how `scale_._continuous`'s `expand` parameter works) could be good?