Added axis keyword to dendrogram function

[jamestwebber]

This fixes issue #1325 (Trac 798). It allows scipy dendrograms to easily be plotted on custom axes, e.g. as part of a larger figure. It does not import pylab at all if an axis is provided, which is useful for those who are using the matplotlib API.

[rgommers]

Overall this looks like a good idea. A couple of comments:

The axis argument should be named ax, because that's how this argument is typically called. axis normally means the axis of the input array over which to apply a certain operation (np.mean(x, axis=1)).

A test should be added for this keyword. Matplotlib is an optional dependency, so the test should only run if MPL is installed. You can look at the stats.probplot test as an example for how to add such a test:
https://github.com/scipy/scipy/blob/master/scipy/stats/tests/test_morestats.py#L17
https://github.com/scipy/scipy/blob/master/scipy/stats/tests/test_morestats.py#L436

[rgommers]

TravisCI failure is unrelated, can be ignored.

[jamestwebber]

Changing from axis to ax is easy enough to do. 'axis' was used throughout the existing code so I went with it.

I wasn't sure how to test a plot, it looks like (in the example you pointed me to) there's no test for the correct output, just to make sure it doesn't fail? I can add that to the existing cluster tests.

[rgommers]

Indeed, checking that it doesn't fail. Comparing graphics output is done by the MPL test suite, but is hard and overkill here.

[jamestwebber]

The interface for dendrogram is slightly different than the one for probplot (and it seems out of scope to try to change that). I didn't compare the output of the two different calls because there isn't the option to pass in pyplot instead of an axis.

[coveralls]

Changes Unknown when pulling d229aa3 on jamestwebber:patch-1 into * on scipy:master*.

[coveralls]

Changes Unknown when pulling d229aa3 on jamestwebber:patch-1 into * on scipy:master*.

[coveralls]

Changes Unknown when pulling bf2ec22 on jamestwebber:patch-1 into * on scipy:master*.

[rgommers]

Test and doc addition look good. PR merged in dc7555b. Thanks @jamestwebber

[rgommers]

Note that I did edit your commit messages to prepend the standard abbreviations listed at http://docs.scipy.org/doc/numpy-dev/dev/gitwash/development_workflow.html#writing-the-commit-message.