matplotlib Code

Update specgram() to use the new keyword arguments and clean up docstring.

Factor out common core of psd(), csd(), and specgram() into _spectral_helper() function. This allows all of them to have the same calling signature and capabilities and to have the code in a single location.

Update cohere() in mlab and Axes method to match new psd() and csd() parameters. Update docs.

Make mlab.psd() call mlab.csd() instead of duplicating 95% of the code. Tweak csd() to check if x and y are the same and avoid duplicating the work if so.

Update the axes.csd() to match the new options added to mlab.csd(). Move some of the docs from axes.psd() to the common place in mlab.

Update the mlab.csd() to match the new options added to mlab.psd(). Factor out the keyword argument docs so that they can be shared between the two functions.

Slight whitespace cleanup.

Update the axes.psd() method to reflect the new options available in mlab.psd. Add an example to show how the options change things.

Add 'pad_to' and 'sides' parameters to mlab.psd() to allow controlling of zero padding and returning of negative frequency components, respecitively. These are added in a way that does not change the API.

Fix autoscaling problem caused by axhline and axvline