Scipy2013 Sprint: Cleaning F/C example

I cleaned the F/C example as suggested in the Scipy 2013 sprint. I had the help of @leouieda on this. For more information, please see #2181.
matplotlib · tacaswell · Feb 10, 2014 · Feb 2, 2014 · Feb 2, 2014 · 53d68db9fa3464b37781ef35190a4e54ef9d2fe0
commit 53d68db9fa3464b37781ef35190a4e54ef9d2fe0
diff --git a/examples/api/fahrenheit_celsius_scales.py b/examples/api/fahrenheit_celsius_scales.py
diff --git a/examples/subplots_axes_and_figures/fahrenheit_celsius_scales.py b/examples/subplots_axes_and_figures/fahrenheit_celsius_scales.py
@@ -0,0 +1,37 @@
+"""
+Demo of how to display two scales on the left and right y axis.
+
+This example uses the Fahrenheit and Celsius scales.
+"""
+import matplotlib.pyplot as plt
+import numpy as np
+
+
+def fahrenheit2celsius(temp):
+    """
+    Returns temperature in Celsius.
+    """
+    return (5. / 9.) * (temp - 32)
+
+
+def convert_ax_c_to_celsius(ax_f):
+    """
+    Update second axis according with first axis.
+    """
+    y1, y2 = ax_f.get_ylim()
+    ax_c.set_ylim(fahrenheit2celsius(y1), fahrenheit2celsius(y2))
+    ax_c.figure.canvas.draw()
+
+fig, ax_f = plt.subplots()
+ax_c = ax_f.twinx()
+
+# automatically update ylim of ax2 when ylim of ax1 changes.
+ax_f.callbacks.connect("ylim_changed", convert_ax_c_to_celsius)
+ax_f.plot(np.linspace(-40, 120, 100))
+ax_f.set_xlim(0, 100)
+
+ax_f.set_title('Two scales: Fahrenheit and Celsius')
+ax_f.set_ylabel('Fahrenheit')
+ax_c.set_ylabel('Celsius')
+
+plt.show()