fix issues with round polar plots, add hexbin_demo.py example

examples/README

@@ -144,3 +144,5 @@ ploticos.py demonstrates plotting on unstructured grids.
 
 lic_demo.py shows how to use vectorplot scikit to visualize vector fields with
 Line Integral Convolutions (LIC).
+
+hexbin_demo.py shows how to plot scattered data using matplotlib hexbin function.

examples/hexbin_demo.py

@@ -0,0 +1,62 @@
+# example showing how to plot scattered data with hexbin.
+from numpy.random import uniform
+import matplotlib.pyplot as plt
+import numpy as np
+from mpl_toolkits.basemap import Basemap
+
+# create north polar stereographic basemap
+m = Basemap(lon_0=270, boundinglat=20, projection='npstere',round=True)
+
+# number of points to plot.
+npts = 10000
+bins = 40
+# generate random points on a sphere,
+# so that every small area on the sphere is expected
+# to have the same number of points.
+# http://mathworld.wolfram.com/SpherePointPicking.html
+u = uniform(0.,1.,size=npts)
+v = uniform(0.,1.,size=npts)
+lons1 = 360.*u
+lats1 = (180./np.pi)*np.arccos(2*v-1) - 90.
+# toss points outside of map region.
+lats = np.compress(lats1 > 20, lats1)
+lons = np.compress(lats1 > 20, lons1)
+# convert to map projection coordinates.
+x, y = m(lons, lats)
+# function to plot at those points.
+xscaled = 4.*(x-0.5*(m.xmax-m.xmin))/m.xmax
+yscaled = 4.*(y-0.5*(m.ymax-m.ymin))/m.ymax
+z = xscaled*np.exp(-xscaled**2-yscaled**2)
+
+# make plot using hexbin
+fig = plt.figure(figsize=(12,5))
+ax = fig.add_subplot(121)
+CS = plt.hexbin(x,y,C=z,gridsize=bins,cmap=plt.cm.jet)
+# draw coastlines, lat/lon lines.
+m.drawcoastlines()
+m.drawparallels(np.arange(0,81,20))
+m.drawmeridians(np.arange(-180,181,60))
+m.colorbar() # draw colorbar
+plt.title('hexbin demo')
+
+# use histogram2d instead of hexbin.
+ax = fig.add_subplot(122)
+#m = Basemap(lon_0=270, boundinglat=20, projection='npstere',round=True)
+bincount, xedges, yedges = np.histogram2d(x, y, bins=bins)
+mask = bincount == 0
+# reset zero values to one to avoid divide-by-zero
+bincount = np.where(bincount == 0, 1, bincount)
+H, xedges, yedges = np.histogram2d(x, y, bins=bins, weights=z)
+H = np.ma.masked_where(mask, H/bincount)
+# set color of masked values to white (hexbin does this by default)
+palette = plt.cm.jet
+palette.set_bad('white', 1.0)
+CS = m.pcolormesh(xedges,yedges,H.T,shading='flat',cmap=palette)
+# draw coastlines, lat/lon lines.
+m.drawcoastlines()
+m.drawparallels(np.arange(0,81,20))
+m.drawmeridians(np.arange(-180,181,60))
+m.colorbar() # draw colorbar
+plt.title('histogram2d demo')
+
+plt.show()