Jamie has informed me of numpy's numpy.savetxt() method, which is exactly what I want for this situation (human-readable text storage of an array).  So, I will now be using:

      # outfile is the name of the .png graph.  data is the array with our desired data.
      numpy.savetxt(outfile + '.dat', data)

to save the data.  I can later retrieve it with numpy.loadtxt()