Improved drawing in plot_rag_draw.py example (#1872)

Improved drawing in draw_rag example; changed draw_rag to show_rag, made it return ScalarMappable to draw colorbars; added magma colormap

doc/examples/segmentation/plot_rag_draw.py

@@ -8,33 +8,25 @@ the `rag_draw` method.
 """
 from skimage import data, segmentation
 from skimage.future import graph
-from skimage.util.colormap import viridis
-from matplotlib import pyplot as plt, colors
+from matplotlib import pyplot as plt
 
 
 img = data.coffee()
 labels = segmentation.slic(img, compactness=30, n_segments=400)
 g = graph.rag_mean_color(img, labels)
 
-out = graph.draw_rag(labels, g, img)
-plt.figure()
-plt.title("RAG with all edges shown in green.")
-plt.imshow(out)
 
-# The color palette used was taken from
-# http://www.colorcombos.com/color-schemes/2/ColorCombo2.html
-cmap = colors.ListedColormap(['#6599FF', '#ff9900'])
-out = graph.draw_rag(labels, g, img, node_color="#ffde00", colormap=cmap,
-                     thresh=30, desaturate=True)
-plt.figure()
-plt.title("RAG with edge weights less than 30, color "
-          "mapped between blue and orange.")
-plt.imshow(out)
+fig, ax = plt.subplots()
+ax.set_title('RAG drawn with default settings')
+lc = graph.show_rag(labels, g, img, ax=ax)
+# fraction specifies the fraction of the area of the plot that will be used to
+# draw the colorbar
+plt.colorbar(lc, fraction=0.03)
 
-plt.figure()
-plt.title("All edges drawn with viridis colormap")
-out = graph.draw_rag(labels, g, img, colormap=viridis,
-                     desaturate=True)
+fig, ax = plt.subplots()
+ax.set_title('RAG drawn with grayscale image and viridis colormap')
+lc = graph.show_rag(labels, g, img, img_cmap='gray', edge_cmap='viridis',
+                    ax=ax)
+plt.colorbar(lc, fraction=0.03)
 
-plt.imshow(out)
 plt.show()