subplot(): fix ystart/yend double flip

[alyst]

This is a small cleanup to the subplot() grid layout logic: when defining the new domains of Y axis, get_domains() sets the new ystart > yend, but later in subplot()/reposition() the ystart and yend are reversed to do the proper transformation.
With the fix the logic of handling both X and Y axes is unified: *start is always less than *end, the axis domain is always from *start to *end.