How to make offset axes in matplotlib?

I want to create two x-axes, one 开发者_如何学JAVAbelow the other.

I believe I want to base it off this example (which looks like this), but in particular how do I draw this outside (below) the original axes region, and set major_ticks and major_ticklabels?

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This example, http://matplotlib.sourceforge.net/examples/pylab_examples/spine_placement_demo.html shows how to move the spines outwards. The rest depends on what sort of relation do you want of your x-spines (do you want them linked or do you want them independent of each other).