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

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Merged
merged 3 commits into from
Jan 23, 2018
Merged

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

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5dcab5e
If a value to convert doesn't have units, just return
dstansby Jan 9, 2018
ca00cca
Allow custom unit class to not be iterable
dstansby Jan 9, 2018
4f25759
Actually make units possibly not iterable
dstansby Jan 9, 2018
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4 changes: 4 additions & 0 deletions lib/matplotlib/axis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1498,6 +1498,10 @@ def have_units(self):
return self.converter is not None or self.units is not None

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

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

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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?)

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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.

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(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) :-))

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

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Was there a decision on supporting mixed types in categorical?

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Just strings, according to #10212

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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.

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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.

return x

if self.converter is None:
self.converter = munits.registry.get_converter(x)

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2 changes: 0 additions & 2 deletions lib/matplotlib/dates.py
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Expand Up @@ -1738,8 +1738,6 @@ def convert(value, unit, axis):

The *unit* and *axis* arguments are not used.
"""
if units.ConversionInterface.is_numlike(value):
return value
return date2num(value)

@staticmethod
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5 changes: 4 additions & 1 deletion lib/matplotlib/tests/test_units.py
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Expand Up @@ -31,7 +31,10 @@ def __getattr__(self, attr):
return getattr(self.magnitude, attr)

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

def __array__(self):
return np.asarray(self.magnitude)
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