Tweaks to mplot3d.polys3d_demo example: change the functionality by a…

…dding extra zero vertices instead of zeroing out the first and last elements of the random data, plus reorganization/refactoring and comments.

examples/mplot3d/polys3d_demo.py

@@ -1,35 +1,54 @@
+'''
+Demonstrate how to create semi-transparent polygons which fill the space
+under a line graph, creating a sort of 'jagged stained glass' effect.
+'''
+
 from mpl_toolkits.mplot3d import Axes3D
 from matplotlib.collections import PolyCollection
 from matplotlib.colors import colorConverter
 import matplotlib.pyplot as plt
 import numpy as np
 
 
+def cc(arg):
+    '''
+    Shorthand to convert 'named' colours to rgba format at 60% opacity.
+    '''
+    return colorConverter.to_rgba(arg, alpha=0.6)
+
+
+def polygon_under_graph(xlist, ylist):
+    '''
+    Construct the vertex list which defines the polygon filling the space under
+    the (xlist, ylist) line graph.  Assumes the xs are in ascending order.
+    '''
+    return [(xlist[0], 0.)] + list(zip(xlist, ylist)) + [(xlist[-1], 0.)]
+
+
 fig = plt.figure()
 ax = fig.gca(projection='3d')
 
+# Make verts a list, verts[i] will be a list of (x,y) pairs defining polygon i
+verts = []
+
+# Set up the x sequence
+xs = np.linspace(0., 10., 26)
 
-def cc(arg):
-    return colorConverter.to_rgba(arg, alpha=0.6)
+# The ith polygon will appear on the plane y = zs[i]
+zs = range(4)
 
-xs = np.arange(0, 10, 0.4)
-verts = []
-zs = [0.0, 1.0, 2.0, 3.0]
-for z in zs:
+for i in zs:
     ys = np.random.rand(len(xs))
-    ys[0], ys[-1] = 0, 0
-    verts.append(list(zip(xs, ys)))
+    verts.append(polygon_under_graph(xs, ys))
 
-poly = PolyCollection(verts, facecolors=[cc('r'), cc('g'), cc('b'),
-                                         cc('y')])
-poly.set_alpha(0.7)
+poly = PolyCollection(verts, facecolors=[cc('r'), cc('g'), cc('b'), cc('y')])
 ax.add_collection3d(poly, zs=zs, zdir='y')
 
 ax.set_xlabel('X')
-ax.set_xlim3d(0, 10)
 ax.set_ylabel('Y')
-ax.set_ylim3d(-1, 4)
 ax.set_zlabel('Z')
+ax.set_xlim3d(0, 10)
+ax.set_ylim3d(-1, 4)
 ax.set_zlim3d(0, 1)
 
 plt.show()