Unfortunately, `m=a+1` is not *exactly* right, since it would need to be `\log(m-1)` in the denominator for that to be true.  What I really mean is that the scalings used by ds9 and aplpy can be very nearly equivalent, but with aplpy you specify `vmid` and not `a`, so it takes some user-end math to make images appear identical.

But yes, I would find it very helpful to be able to directly specify `normalize.midpoint` via keyword.