r.horizon: add polar plot example (merge from trunk, https://trac.osgeo.org/grass/changeset/65706)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@66355 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.horizon/r.horizon.html

@@ -170,6 +170,28 @@ Test point near high way intersection (North Carolina sample dataset)
 Horizon angles for test point (CCW from East)
 </center>
 
+<p>We can plot horizon in polar coordinates using Matplotlib in Python:
+<div class="code"><pre>
+import numpy as np
+import matplotlib.pyplot as plt
+
+horizon = np.genfromtxt('horizon.csv', delimiter=',')
+horizon = horizon[1:, :]
+
+ax = plt.subplot(111, polar=True)
+bars = ax.plot(horizon[:, 0] / 180 * np.pi,
+               (90 - horizon[:, 1]) / 180 * np.pi)
+# uncomment the 2 following lines when using -c flag
+# ax.set_theta_direction(-1)
+# ax.set_theta_zero_location('N')
+plt.show()
+</pre></div>
+
+<center>
+<img src="rhorizon_polar_plot.png"><br>
+Horizon plot in polar coordinates.
+</center>
+
 <h3>Raster map mode</h3>
 
 Raster map mode (output maps "horangle*" become input for <em>r.sun</em>):