Re: [matplotlib-devel] Empty scatter plot

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

On Tue, Feb 15, 2011 at 11:54 AM, Eric Firing <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