secondary y-axis

And can I make one legend that includes the plots from both

    > axes?

Save a list of lines and a list of labels and then call

    ax.legend(lines, labels)

lines and labels can be from different axes; eg,

    lines = []
    lines.extend( ax1.plot(something))
    lines.extend( ax2.plot(something_else))

See also the figure legend capability in
http://matplotlib.sf.net/matplotlib.figure.html

    > Thanks for your help John. I think I am getting close to a
    > really nice graph with a lot of useful information on it.

Your welcome -- good luck.

JDH