Switch correctly to the user's default matplotlib backend after inline.

[fperez]

If '%matplotlib inline' was called first, we'd incorrectly revert to
inline when plain '%matplotlib' was called, instead of loading the
user's default GUI. If the user called '%matplotlib' first (without
'inline') it worked correctly, but not in the other order.

The fix is to read the backend from the original defaults, not from
the runtime data structure.

[minrk]

Seems sensible enough - has rcParamsOrig been around long enough that we don't need a check on older matplotlibs (I just don't know).

[fperez]

It's been there in mpl since 1.1.0, which was released in February 2012. Mpl 1.0 didn't have it.

I'm inclined not to cruft up our code with too many protections for old third-party libraries in general. What do you think? I just hate layers and layers of try/except for that kind of thing, which then take years to clean up (since nobody is going to remember to go back in a year to remove it).

[Carreau]

We should probably think of a mechanism that warn/raise on IPython version bump to force us to clean things.

Something along :

with deprecate(warn=2.0, error=3.0):
    # log.warn if IPython version > 2.0
    # log.error (eventually die) if IPython > 3.0
    (do stuff)

Can do the same with decorator.

This would also make our plan more visible for people tweeking the code.

[minrk]

It seems pretty significant that this change means %matplotlib won't work on matplotlib 1.0.

[ellisonbg]

Yes, that is probably too aggressive.

[fperez]

I'll add info about 1.1 being required (comment in block, a box in the docs).

[fperez]

I think this one's good to go, as best I can tell. Let me know if you guys spot anything else.