I have a number of questions, but I'll start with one.  Consider the following code:

import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
x = np.arange(1, 6)
plt.plot(x, x*1.5)
plt.show()
s = raw_input('Enter anything to continue:')
plt.show()
print 'The end'

Question:  The first plt.show() launches an interactive window.  Control returns to the script upon its close.  The second plt.show() does nothing that I can see.  Why?

Ubuntu 11.04, Python 2.7.1, Matplotlib 0.99.3