Logarithmic scale + histograms to matplotlib text

Hello,

as my colleague and me are teaching "Scientific computing in Python" lessons, we sometimes base our lectures directly on your notebooks (they're excellent). Last week I taught about Matplotlib and two ideas for extending your lecture jumped into my mind.

1) Logarithmic scale - scientists use this very often and require the plotting SW to do that for them
2) Histograms - again very useful feature (not for all fields though)

Of course you have to keep your text reasonably short, so please just consider these the two additions in separate commits.

I included only the relevant chunks as running the notebook by default recalculates all outputs and images (that are not same on the pixel level).

Best regards,
Jan