Save figure for "plot two strain ts" script

To improve compatibility with MyBinder, this commit swaps interactive plotting with TkAgg + plt.show() to saving figure output as PNG with Agg + plt.savefig().

models/exercise/README.md

@@ -17,7 +17,9 @@ Subscript indices refer to the status with respect to strains 1 and 2, respectiv
 
 Compare answers with your neighbors, and be sure to raise any interesting points of disagreement (or agreement or confusion) with the class.
 
-For this next section you'll need to download a Python script. You can download it [here](https://github.com/trvrb/sismid/blob/master/models/exercise/exercise.zip?raw=true). Download this and uncompress it.
+For this next section you'll need to run a Python script.
+
+This can either be downloaded [here](https://github.com/trvrb/sismid/blob/master/models/exercise/exercise.zip?raw=true) and run locally or can be run directly from within MyBinder at [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/trvrb/sismid/HEAD). This script outputs the figure `fig_plot_two_strain_ts.png`.
 
 Let's assume we think this model is a pretty good starting point for testing hypotheses.
 We'll numerically integrate these equations to see what they predict.

models/exercise/plot_two_strain_ts.py

@@ -7,8 +7,8 @@
 import csv
 import numpy as np
 import matplotlib as mpl
-mpl.use('TkAgg')
-from matplotlib.font_manager import FontProperties 
+mpl.use('Agg')
+from matplotlib.font_manager import FontProperties
 import matplotlib.pyplot as plt
 from two_strain import *
 
@@ -85,5 +85,5 @@
 plt.ylabel('Infected 2')
 plt.legend(loc=1, prop=FontProperties(size='smaller'))
 plt.savefig("time_series_" + str(run_num) + ".png")
-plt.show()
+plt.savefig("fig_plot_two_strain_ts.png", dpi=200)
 plt.close()