double x-axis with defined xtick and inverse order

Dear all,

I would like to plot a figure with double x-axes.
Unfortunately, it seems the ticks of the second x axis do not fit with what
I want.

Here the example:

import numpy as np
import matplotlib.pyplot as plt

MM = np.array([ 1., 2., 4., 8., 16., 32.])
SS = np.array([64., 32., 16., 8., 4., 2.])
RR = np.array([2813, 1544, 895, 562, 459,409])

ax = plt.subplot(111)
ax.plot(MM, RR ,'-r')

ax2 = ax.twiny()
ax2.plot(MM, RR, 'none')

As it can be noticed, the value 32 in the ax2 does not correspond to the
second value but is put in the middle point of ax2. I would like to have it
in the same vertical line of the second value of MM. This of course not
only for 32 but for the value in SS. Basically I would like to have the
main axis according to MM and a second ax with SS value in the same line of
MM values.

I hope I made myself clear.

really really thanks and,
best regards

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