Don't convert numbers plotted on an axis with units by dstansby · Pull Request #10206 · matplotlib/matplotlib · GitHub

I've been playing around trying to get Astropy units to play nicely with Matpltolib recently. One of the things I've realised is that some of the datetime plotting capabilities rely on the datetime converter just passing through numbers without attempting to convert them. (the specific example where I came across this is axhline/axvline when the x and y units are different).

This PR means custom converters do not have to manually pass through floats (e.g.), but instead it is automatically done at "convert time". I think this makes explicit what @efiring said in this comment about always letting unit-less data being plotted on an axis with units.

Also pinging @jklymak because this probably affects your converter PR.

🤦‍♂️ the most useless if statement in the world.

Right now the assigned converter deals with the floats.

This change means that a user can never create a converter that accepts numlike vectors and does something with them. They'd always have to wrap it in a class that is_numlike doesn't think is numlike.

I guess I'm OK w/ that restriction, and I don't think it harms any of our current converters. But, I'm not 100% sure what the rest of the world is doing.

BTW pinging @tacaswell, @anntzer and @story645 as they are working on categoricals where unit handling has come up a lot

Hmm, you make a good point. If the decision is to allow plotting numbers on an axis with units, then this PR is the way to go. (In principle I'm -1 on that, but I wasn't on the call so I'm guessing there's good reasons for allowing it?)

I think there are good reasons for both.

@efiring 's example was: imagine you plot five categories, and then want a line at x=0.5. Thats hard to do if we have locked out x to be only categories.

I'm not 100% convinced we can't come up w/ a way to let the user force the units, but still retain the lockout by default.

(I wasn't really "working on categoricals", just trying to write some version that's reasonably robust and doesn't do too silly things (which is quite hard) :-))

But axvline('5') would work and line up with the 5 category right?

Was there a decision on supporting mixed types in categorical?

Just strings, according to #10212

Axvline plots a line at an x in data coordinates. If you want that half-way between two categorical values, you have to be able to specify that location in numeric data coordinates. If you know how categoricals are mapped to numbers, and you are allowed to plot in numeric data coordinates, this is easy. Fundamentally, all plotting is done in floating point coordinates. Plotting something in "units" should not prevent the user from positioning something directly in the numbers to which those units map--and from doing so efficiently.

Regarding your example, it should be well-documented--if it isn't already--that categoricals map sequentially to integers starting from zero. Knowing that, the user can easily calculate that the point half-way between your '2' and '3' (which more realistically might be 'dogs' and 'cats' or "instrument 872" and "instrument 253") is 1.5.

Since the consensus seems to be "always allow numbers on unitised axes", I'm going to provocatively tag this as release critical, since it's a fairly minor change but would massively help plotting non-datetime units.

... Units really need a MEP at some point 😉

But, my opinion still is:

We should lock out new converters, and that all axis instances should be "unitised" on their first usage. i.e. if your axis is assigned to the Datetime Converter because you passed a datetime object to it (first), or explictly set it, then its stuck there (until you explictly unbind it somehow).
If the converter wants to allow subsequent floats or numlike, it should be allowed to do that. But, it should also be allowed to block them. i.e. I think all subsequent calls should be routed through the converter.

Decided on call that the use case above can be considered when the use case above is actually needed....

dstansby added 2 commits January 9, 2018 20:22

If a value to convert doesn't have units, just return

5dcab5e

Allow custom unit class to not be iterable

ca00cca

dstansby added the topic: units and array ducktypes label Jan 9, 2018

dstansby commented Jan 9, 2018

View reviewed changes

Actually make units possibly not iterable

4f25759

jklymak reviewed Jan 9, 2018

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dstansby added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Jan 15, 2018

jklymak added this to the v2.2 milestone Jan 22, 2018

jklymak approved these changes Jan 22, 2018

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jklymak added the topic: categorical label Jan 22, 2018

efiring approved these changes Jan 22, 2018

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dstansby merged commit 296b5ff into matplotlib:master Jan 23, 2018

dstansby mentioned this pull request Jan 23, 2018

Allow lists of quantities to be converted in Matplotlib unit converter astropy/astropy#7037

Merged

dstansby mentioned this pull request Feb 2, 2018

Error when using quantities when plotting errorbars #8871

Closed

dstansby deleted the dont-convert-numbers branch February 4, 2018 19:15

QuLogic modified the milestones: needs sorting, v2.2.0 Feb 12, 2018

dstansby mentioned this pull request May 29, 2018

Commit #efaa3b3 breaks rendering of legends astropy/astropy#7504

Closed

tacaswell mentioned this pull request Jan 3, 2021

Add public API for setting an axis unit converter #19229

Closed

richeshgupta mentioned this pull request Sep 23, 2026

SWR matplotlib__matplotlib-10206 razorpaytest/matplotlib#48

Open

+                      if iterable(self.magnitude):
+                          return Quantity(self.magnitude[item], self.units)
+                      else:
+                          return Quantity(self.magnitude[item], self.units)

                   def convert_units(self, x):
+                      # If x is already a number, doesn't need converting
+                      if munits.ConversionInterface.is_numlike(x):

Don't convert numbers plotted on an axis with units by dstansby · Pull Request #10206 · matplotlib/matplotlib · GitHub

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Don't convert numbers plotted on an axis with units - #10206

Don't convert numbers plotted on an axis with units#10206dstansby merged 3 commits intomatplotlib:mastermatplotlib/matplotlib:masterfrom dstansby:dont-convert-numbersdstansby/matplotlib:dont-convert-numbersCopy head branch name to clipboard

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dstansby commented Jan 9, 2018

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dstansby commented Jan 15, 2018

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jklymak commented Jan 15, 2018

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jklymak commented Jan 22, 2018

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Don't convert numbers plotted on an axis with units#10206
dstansby merged 3 commits into
matplotlib:masterfrom
dstansby:dont-convert-numbers