On Tue, Feb 15, 2011 at 2:05 PM, Benjamin Root <ben.r...@ou.edu> wrote:
> On Tue, Feb 15, 2011 at 1:41 PM, Benjamin Root <ben.r...@ou.edu> wrote:
>
>>
>>
>> On Tue, Feb 15, 2011 at 1:17 PM, Eric Firing <efir...@hawaii.edu> wrote:
>>
>>> On 02/15/2011 08:50 AM, Benjamin Root wrote:
>>>
>>>> On Tue, Feb 15, 2011 at 12:19 PM, Benjamin Root <ben.r...@ou.edu
>>>> <mailto:ben.r...@ou.edu>> wrote:
>>>>
>>>> On Tue, Feb 15, 2011 at 11:54 AM, Eric Firing <efir...@hawaii.edu
>>>> <mailto:efir...@hawaii.edu>> wrote:
>>>>
>>>> On 02/15/2011 07:40 AM, Benjamin Root wrote:
>>>> > I have come across a little inconsistency that was unexpected
>>>> in the
>>>> > matplotlib API. The following is perfectly valid:
>>>> >
>>>> > import matplotlib.pyplot as plt
>>>> > plt.plot([], [])
>>>> > plt.show()
>>>> >
>>>> >
>>>> > However, this is not valid:
>>>> >
>>>> > import matplotlib.pyplot as plt
>>>> > plt.scatter([], [])
>>>> > plt.show()
>>>> >
>>>> >
>>>> > The immediate issue that comes up in scatter is that it
>>>> attempts to find
>>>> > min/max of the input data for the purpose of autoscaling
>>>> (this can
>>>> > probably be done better by just using set_xmargin(0.05) and
>>>> > set_ymargin(0.05)). This can easily be bypassed with an if
>>>> statement.
>>>> > However, then we discover that polygon collection do not like
>>>> having
>>>> > empty offsets, which leads to a failure in the affine
>>>> transformation.
>>>> >
>>>> > So, the question is, is this a bug or a feature? I
>>>> personally believe
>>>> > that empty data is a perfectly valid scenario and given that
>>>> other
>>>> > matplotlib functions handle it gracefully, we should make the
>>>> > collections object more friendly to empty data.
>>>>
>>>> I agree; a call with empty data should simply not plot anything.
>>>>
>>>> Eric
>>>>
>>>>
>>>> Digging further, it appears that the problem is in _path.cpp for
>>>> _path_module::affine_transform() which explicitly checks for an
>>>> empty vertices array and throws an exception if it is empty.
>>>>
>>>> So, do we want to make _path.cpp empty-friendly or should we just
>>>> make empty collections objects just avoid doing anything that
>>>> requires doing an affine transform?
>>>>
>>>> Ben Root
>>>>
>>>>
>>>>
>>>> Ok, some more digging deepens the mystery. While an empty-friendly
>>>> _path.cpp would be nice, it appears that the collections and axes
>>>> objects are already doing all it can to avoid doing transforms for empty
>>>> collections.
>>>>
>>>> However, it appears that the supposedly empty collection object from
>>>> scatter([], []) is not really empty. Its self._paths member contains a
>>>> list of unit_circle() from Path. This is also the case for
>>>> EllipseCollection. Meanwhile, LineCollection and PatchCollection
>>>> initializes their self._paths in accordance to their given data.
>>>>
>>>
>>> One way to solve the problem would be to start each draw() method with a
>>> short-circuit return in case there is nothing to draw. It would be needed
>>> only for classes for which empty self._paths is not a valid test. So for
>>> CircleCollection it would be:
>>>
>>> @allow_rasterization
>>> def draw(self, renderer):
>>> # sizes is the area of the circle circumscribing the polygon
>>> # in points^2
>>> if len(self._sizes) == 0:
>>> return
>>> self._transforms = [
>>> transforms.Affine2D().scale(
>>> (np.sqrt(x) * self.figure.dpi / 72.0) / np.sqrt(np.pi))
>>> for x in self._sizes]
>>> return Collection.draw(self, renderer)
>>>
>>> (Collection.draw returns nothing, so there is no inconsistency in the
>>> return value.)
>>>
>>> Alternatively, it looks like maybe an empty self._transforms could be
>>> used in a short-circuit test at the start of Collection.draw.
>>>
>>> Eric
>>>
>>>
>>>
>>>> Ben Root
>>>>
>>>>
>>>
>> That wouldn't completely solve the problem. Affine transforms are being
>> done for get_datalim() as well. The other issue (and I see that I mixed
>> this up myself) is the assumption elsewhere that a non-empty self._path
>> attribute means that there is something to plot. This is an assumption that
>> is made in axes.py on line 1413 and it is an invalid assumption.
>>
>> As for your proposed solution in draw(), I prefer short-circuiting in
>> Collections.draw(). This makes for less work for new Collection subclasses.
>>
>> Ben Root
>>
>
>
> Working from an assumption that we can't possibly find all bad/broken
> assumptions in mpl, I decided to go the route of Eric and make the
> Collections object as robust as possible. I have attached a patch that
> should enable empty scatter plots in matplotlib and enable empty Collections
> as well.
>
> This has not been fully tested yet. Please be brutal!
>
> Ben Root
>
>
>
Heh, looks like that patch breaks the plotting of errorbars (the line
connecting the center point to the error caps is missing).
I will see about fixing that.
Ben Root
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