BUG: Add subplot spec eq

[tacaswell]

See: http://stackoverflow.com/questions/20017314/how-are-existing-subplot-axes-retrieved-using-subplot2grid-gridspec-in-matplotli

in short,

gs = gridspec.GridSpec(2,1)
In [220]: gs[0, 0] == gs[0, 0]
Out[220]: False

which breaks the logic of calling ax = fig.add_subplot(gs[0,0]) multiple times.

[tacaswell]

And this seems to break all sorts of things and I have no idea why.

[mdboom]

I suspect a lot of these failures can be fixed by putting isinstance(other, SubplotSpec) (or hasattr(other, '_gridspec') if we'd rather do duck typing) to the beginning of the expression. I don't know if that will fix everything else.

[tacaswell]

The other way to do this would be to wrap the all in a try...except not sure which way is preferred.

[Tillsten]

There is sometimes a real need of getting a different axes at the same place: twinx etc.
So this should be at least doable with a keyword, which means i am not sure which default behavior makes
more sense.

[Tillsten]

Ahh, i see that doable by #2515, than nvm :)

[tacaswell]

And, again I get errors on python 3 that I do not really understand. Lets see if adding __hash__ makes things better....

@Tillsten I am confused, #2515 is change in mpl_toolkits which makes divided axes play nice with twinx and is (I think) completely orthogonal to this one.

[Tillsten]

@tacswell Oh, i maybe i read #2515 too fast. I used the old behavior to make my own twinx with gs, but now it seems it was never necessary. Sorry for the noise.

[pelson]

@tacaswell - nice easy one to test. If you can add a unit test, I will merge. 👍

[tacaswell]

@pelson test added.

[duckrojo]

This is an old bug, but it seems it has not been corrected? I still get a new axes instances instead of referring an existing axes; or am I missing anything?

[tacaswell]

@duckrojo what version of mpl are you using?

The tests added in this PR are still in the code base (and being run by our CI) Can you provide a MWE of what is broken?