plotting support for hourly, minutely, secondly frequencies

[mattknox_ca]

Currently plotlib.py does not support the hourly, minutely, and secondly frequencies. This is something that should be added.

[thelusiv]

I could very much use these features (especially minutely) in a project I'm currently working on. In fact I'm probably going to start implementing them myself very soon. Please email me at thelusiv()gmail-com if you have a patch or suggestions for how to implement.

[timmichelsen]

In the mean time please raise a exception when trying to plott a ferquency lower that the ones currently supported. This could help.