text: """
  Contour plots of unstructured triangular grids.
  """
  import matplotlib.pyplot as plt
  import matplotlib.tri as tri
  import numpy as np
  import math
  
  # Creating a Triangulation without specifying the triangles results in the
  # Delaunay triangulation of the points.
  
  # First create the x and y coordinates of the points.
  n_angles = 48
  n_radii = 8
  min_radius = 0.25
  radii = np.linspace(min_radius, 0.95, n_radii)
  
  angles = np.linspace(0, 2*math.pi, n_angles, endpoint=False)
  angles = np.repeat(angles[..., np.newaxis], n_radii, axis=1)
  angles[:, 1::2] += math.pi/n_angles
  
  x = (radii*np.cos(angles)).flatten()
  y = (radii*np.sin(angles)).flatten()
  z = (np.cos(radii)*np.cos(angles*3.0)).flatten()
  
  # Create the Triangulation; no triangles so Delaunay triangulation created.
  triang = tri.Triangulation(x, y)
  
  # Mask off unwanted triangles.
  xmid = x[triang.triangles].mean(axis=1)
  ymid = y[triang.triangles].mean(axis=1)
  mask = np.where(xmid*xmid + ymid*ymid < min_radius*min_radius, 1, 0)
  triang.set_mask(mask)
  
  # pcolor plot.
  plt.figure()
  plt.gca().set_aspect('equal')
  plt.tricontourf(triang, z)
  plt.colorbar()
  plt.tricontour(triang, z, colors='k')
  plt.title('Contour plot of Delaunay triangulation')
  
  # You can specify your own triangulation rather than perform a Delaunay
  # triangulation of the points, where each triangle is given by the indices of
  # the three points that make up the triangle, ordered in either a clockwise or
  # anticlockwise manner.
  
  xy = np.asarray([
      [-0.101, 0.872], [-0.080, 0.883], [-0.069, 0.888], [-0.054, 0.890],
      [-0.045, 0.897], [-0.057, 0.895], [-0.073, 0.900], [-0.087, 0.898],
      [-0.090, 0.904], [-0.069, 0.907], [-0.069, 0.921], [-0.080, 0.919],
      [-0.073, 0.928], [-0.052, 0.930], [-0.048, 0.942], [-0.062, 0.949],
      [-0.054, 0.958], [-0.069, 0.954], [-0.087, 0.952], [-0.087, 0.959],
      [-0.080, 0.966], [-0.085, 0.973], [-0.087, 0.965], [-0.097, 0.965],
      [-0.097, 0.975], [-0.092, 0.984], [-0.101, 0.980], [-0.108, 0.980],
      [-0.104, 0.987], [-0.102, 0.993], [-0.115, 1.001], [-0.099, 0.996],
      [-0.101, 1.007], [-0.090, 1.010], [-0.087, 1.021], [-0.069, 1.021],
      [-0.052, 1.022], [-0.052, 1.017], [-0.069, 1.010], [-0.064, 1.005],
      [-0.048, 1.005], [-0.031, 1.005], [-0.031, 0.996], [-0.040, 0.987],
      [-0.045, 0.980], [-0.052, 0.975], [-0.040, 0.973], [-0.026, 0.968],
      [-0.020, 0.954], [-0.006, 0.947], [ 0.003, 0.935], [ 0.006, 0.926],
      [ 0.005, 0.921], [ 0.022, 0.923], [ 0.033, 0.912], [ 0.029, 0.905],
      [ 0.017, 0.900], [ 0.012, 0.895], [ 0.027, 0.893], [ 0.019, 0.886],
      [ 0.001, 0.883], [-0.012, 0.884], [-0.029, 0.883], [-0.038, 0.879],
      [-0.057, 0.881], [-0.062, 0.876], [-0.078, 0.876], [-0.087, 0.872],
      [-0.030, 0.907], [-0.007, 0.905], [-0.057, 0.916], [-0.025, 0.933],
      [-0.077, 0.990], [-0.059, 0.993]])
  x = np.degrees(xy[:, 0])
  y = np.degrees(xy[:, 1])
  x0 = -5
  y0 = 52
  z = np.exp(-0.01*((x - x0)*(x - x0) + (y - y0)*(y - y0)))
  
  triangles = np.asarray([
      [67, 66,  1], [65,  2, 66], [ 1, 66,  2], [64,  2, 65], [63,  3, 64],
      [60, 59, 57], [ 2, 64,  3], [ 3, 63,  4], [ 0, 67,  1], [62,  4, 63],
      [57, 59, 56], [59, 58, 56], [61, 60, 69], [57, 69, 60], [ 4, 62, 68],
      [ 6,  5,  9], [61, 68, 62], [69, 68, 61], [ 9,  5, 70], [ 6,  8,  7],
      [ 4, 70,  5], [ 8,  6,  9], [56, 69, 57], [69, 56, 52], [70, 10,  9],
      [54, 53, 55], [56, 55, 53], [68, 70,  4], [52, 56, 53], [11, 10, 12],
      [69, 71, 68], [68, 13, 70], [10, 70, 13], [51, 50, 52], [13, 68, 71],
      [52, 71, 69], [12, 10, 13], [71, 52, 50], [71, 14, 13], [50, 49, 71],
      [49, 48, 71], [14, 16, 15], [14, 71, 48], [17, 19, 18], [17, 20, 19],
      [48, 16, 14], [48, 47, 16], [47, 46, 16], [16, 46, 45], [23, 22, 24],
      [21, 24, 22], [17, 16, 45], [20, 17, 45], [21, 25, 24], [27, 26, 28],
      [20, 72, 21], [25, 21, 72], [45, 72, 20], [25, 28, 26], [44, 73, 45],
      [72, 45, 73], [28, 25, 29], [29, 25, 31], [43, 73, 44], [73, 43, 40],
      [72, 73, 39], [72, 31, 25], [42, 40, 43], [31, 30, 29], [39, 73, 40],
      [42, 41, 40], [72, 33, 31], [32, 31, 33], [39, 38, 72], [33, 72, 38],
      [33, 38, 34], [37, 35, 38], [34, 38, 35], [35, 37, 36]])
  
  # Rather than create a Triangulation object, can simply pass x, y and triangles
  # arrays to tripcolor directly.  It would be better to use a Triangulation
  # object if the same triangulation was to be used more than once to save
  # duplicated calculations.
  plt.figure()
  plt.gca().set_aspect('equal')
  plt.tricontourf(x, y, triangles, z)
  plt.colorbar()
  plt.title('Contour plot of user-specified triangulation')
  plt.xlabel('Longitude (degrees)')
  plt.ylabel('Latitude (degrees)')
  
  plt.show()
  