Just found this package that has not been updated in a while, but works so far in Python 2.7.3 (on 64-bit Windows 7, using pretty up-to-date supporting packages):
In [1]: import spc
In [2]: import matplotlib.pyplot as plt
In [3]: x = [25,19,14,17,25,39,49,6,11,19,13,26,24,32,14,19]
In [4]: cc = spc.Spc(x, spc.CHART_X_MR_X)
In [5]: cc.get_chart()
In [6]: plt.show()
Looks like the 6th point is outside the upper control limit...
In [7]: cc.get_violating_points()
Out[7]: {'1 beyond 3*sigma': [6]}
The package is basically a single init.py file that is only a few hundred source lines, and looks to implement more than a dozen charts, including CUSUM.
Lastly, there is a github project worth keeping an eye on: https://github.com/bwghughes/controlchart
