Plotting fails plt.show() and this should work for X-terminals

[texadactyl]

Interactively, blimpy plotting should be able to show a plot. This currently does not work although the plot is saved as a PNG file.

Gross symptom:

UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
  plt.show()

After reviewing the watutil plotting process with Spyder debugger, it appears that:

• Initially (when config.py is imported by waterfall.py), the matplotlib backend is indeed interactive (E.g. "QtAgg").
• Not very long afterwards, when the cmd_tool function is entered, the backend appears to have somehow been changed to "agg".

[texadactyl]

Simple plotting programs have no trouble getting plt.show() to work as expected.

[texadactyl]

Matplotlib backend selection is a mystery! However, I have a work-around solution.

• Get the backend ID when waterfall.py is loaded; save it as a global variable.
• When the cmd_tool function is called, set the backend ID from the global variable.

[wfarah]

Thanks for your help on this @texadactyl !