Deprecate MAXTICKS, Locator.raise_if_exceeds. - #8100

I am conflicted on this.

This limit came in via 0022de2 (and the comment says it is to prevent out of memory).

On one hand, we have had several reports people running into this limit on machines that can handle it and we should not be putting artificial limits on things.

On the other hand, if we raise, users can in principle recover where as if we just let things run out of memory they can not.

On the third hand, rendering more than 1k ticks to the raster formats gets silly, there just are not enough pixels in most cases (and won't go so well with the vector formats either).

I think the option here are:

1. do nothing to code, but document the limit better
2. drop limit all together (this PR)
3. bump up limit (only kicks can down the road)
4. make limit 'smart' based on current figure size + dpi ( + memory?!) which is probably more complicated than we want to deal with.

On the other hand, if we raise, users can in principle recover where as if we just let things run out of memory they can not.

Actually they can't even catch it (I think) because the exception is raised in the GUI thread during draw_idle (try putting try... except RuntimeError around plt.show and see it doesn't work). Additionally the exception is raised again when you click with the mouse (pick -> get_children -> get_major_ticks), which leads to a fatal, aborting error on the Qt5 backend at least (cf #7197).

make limit 'smart' based on current figure size + dpi ( + memory?!) which is probably more complicated than we want to deal with.

We don't do anything for line plots or images (for which there's also a point of diminishing returns), I don't see why tickers should be different.

They can catch it in the context of fig.savefig, I am more worried about non-interactive things silently dying rather vs interactive things.

The difference between user supplied data and ticks is that if they hand us large volumes, it is on them to know what they are doing, but with ticks we do a lot more 'automagically' so need to be more careful.

Fair enough. I have a strong suspicion that the number of people currently doing

try: fig.savefig(...)
except RuntimeError: ...

is pretty low though :-)

I like option 1. The limit was put in for a perfectly good reason, based on experience that is still relevant.

As mentioned above, the limit can only help if one is using something like

try: fig.savefig(...)
except RuntimeError: ...

(afaict, let me know if I missed other possibilities) and actually has a very negative impact (program abort) for programs using the Qt5 backend.

As I said, I doubt there's much code in the wild using that pattern (if you're aware of that possibility you may as well make sure you won't trigger that code path by setting the tickers properly to start with). Unless you can prove me wrong, I would still favor removing the limit.

Still have not reached consensus on this, punting to 3.2

This does couple into the more recent work with logging, this may be a case where we want to change exceptions into log messages?

Closed in favor of #13510.