Re: [matplotlib-devel] Empty scatter plot

[Benjamin R. <ben...@ou...>]

On Tue, Feb 15, 2011 at 2:05 PM, Benjamin Root <ben...@ou...> wrote:

> On Tue, Feb 15, 2011 at 1:41 PM, Benjamin Root <ben...@ou...> wrote:
>
>>
>>
>> On Tue, Feb 15, 2011 at 1:17 PM, Eric Firing <ef...@ha...> wrote:
>>
>>> On 02/15/2011 08:50 AM, Benjamin Root wrote:
>>>
>>>> On Tue, Feb 15, 2011 at 12:19 PM, Benjamin Root <ben...@ou...
>>>> <mailto:ben...@ou...>> wrote:
>>>>
>>>>    On Tue, Feb 15, 2011 at 11:54 AM, Eric Firing <ef...@ha...
>>>>    <mailto:ef...@ha...>> 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