[axis] fix label transform

[williaster]

This PR fixes a bug with the (y-axis) label transform logic. For y-axes you often have a pattern where you set the following domain and range to correct for the upper-right (0,0) svg coord:

domain = [min val, max val];
range = [height, 0] 
// y-axis has `min val` at the bottom and `max val` at the top

If you want to have the min value at the top of the scale you actually invert the range to [0, height]. The current y-label transform assumes the former and this PR fixes it.

Before:

After:

Verified that current transforms still work as expected:

[coveralls]

Coverage remained the same at 27.126% when pulling e8148b7 on williaster:chris--label-transform into a7fbf04 on hshoff:master.

[hshoff]

👍