This has a few weaknesses:
 - For "normal" ticking, it bypasses matplotlib's attempt to place tick marks at round values
 - I haven't tried it yet, but I suspect this will be overridden if the user pans/zooms

I think matplotlib has a utility to supply coordinate tick locator and labeler functions, and these are probably the better option. I don't remember exactly how this works, off-hand. My vote is either to use that approach, or have _update_ticks be a no-op in this PR and open a separate issue to implement it