[matplotlib-devel] Bug in transform argument for some collections?

[Tony Yu <ts...@gm...>]

There seems to be a bug in the "transform" argument for CircleCollection,
EllipseCollection, and RegularPolyCollection. These classes override any
transform passed on instantiation and uses the IdentityTransform instead.

Take, for example, the init method CircleCollection:

#~~~
    def __init__(self, sizes, **kwargs):
        """
        *sizes*
            Gives the area of the circle in points^2

        %(Collection)s
        """
        Collection.__init__(self,**kwargs)
        self._sizes = sizes
        self.set_transform(transforms.IdentityTransform())
        self._paths = [mpath.Path.unit_circle()]
#~~~

If "transform" is passed as a kwarg, it gets updated by the call to
``Collection.__init__`` (this update is done in Artist, which is a parent
class of Collection). But a couple lines after that, the transform is
manually reset to the IdentityTransform. Is this a bug?

As it stands, setting the transform kwarg does nothing (and doesn't
complain), so I have to call the set_transform method after creating the
collection (not a big deal, but it's confusing that the first option
doesn't work).

A couple of notes:
* Artist already defaults to setting Artist._transform (and hence
Collection._transform) to the IdentityTransform.
* Nevertheless, the set_transform line above is *not* a do-nothing line:
not only does set_transform set the _transform attribute, it also sets the
_transformSet attribute.

A few solutions:
* Replace the set_transform line with ``self._transformSet = True``.
(preserves current behavior, without overriding a user-prescribed transform)
* Replace transform passed to self.set_transform with self.get_transform.
(same behavior as above.)
* Just remove the set_transform line. This changes the behavior since
self._transformSet is left False, but I'm not sure what the correct
behavior is.

-Tony