title: imshow and histogram2d: can't get them to work
body: <p>I'm learning Python and this is my first question here. I've read other topics related to the usage of <strong>imshow</strong> but didn't find anything useful. Sorry for my bad English.</p>
  
  <p>I have plotted a set of points here, left graphic:</p>
  
  <p><a href="https://dl.dropbox.com/u/10411539/python/image.png" rel="noreferrer">points (left) and image (right)</a></p>
  
  <p>Now I'd like to see an image of the density of points, so I used <strong>imshow</strong> and <strong>histogram2d</strong>, and I got the image to the right in the previous link.</p>
  
  <p>The image doesn't correspond to the distribution of points. How is this possible? I've followed the instructions in the help and even changed some parameters but nothing worked :( </p>
  
  <p>The code is:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  import matplotlib.cm as cm
  
  j, h, k = np.loadtxt("test.dat", usecols=(2, 4, 6), \
      unpack=True)
  
  # límites
  xmin = -0.5
  xmax =  3.0
  ymin = -0.5
  ymax =  4.0
  
  # colores
  j_h = j - h
  h_k = h - k
  
  # no todas las estrellas son graficadas    
  x1 = 0.5
  y1 = 0.5
  b  = 2.2
  c  = y1 - b * x1
  
  x = y = np.array([])
  
  for xi, yi in zip(h_k, j_h):
      if xi &lt; (yi - c) / b:
          x = np.append(x, xi)
          y = np.append(y, yi)
  
  # gráfico
  fig = plt.figure(figsize=(8, 7))
  
  ax = fig.add_subplot(111)
  #ax.plot(x, y, "go")
  ax.set_xlabel(r"X", fontsize=14)
  ax.set_ylabel(r"Y", fontsize=14)
  ax.axis([xmin, xmax, ymin, ymax])
  
  # imagen
  rango = [[xmin, xmax], [ymin, ymax]]
  binsx = int((xmax - xmin) / 0.05)
  binsy = int((ymax - ymin) / 0.05)
  binsxy = [binsx, binsy]
  
  H, xedges, yedges = np.histogram2d(x, y, range=rango, bins=binsxy)
  
  extent = [yedges[0], yedges[-1], xedges[0], xedges[-1]]
  cp = ax.imshow(H, interpolation='bilinear', extent=extent, cmap=cm.jet)
  fig.colorbar(cp)
  
  plt.show()
  </code></pre>
  
  <p>The links for the data used is here:</p>
  
  <p><a href="https://dl.dropbox.com/u/10411539/python/test.dat" rel="noreferrer">https://dl.dropbox.com/u/10411539/python/test.dat</a></p>
  
  <p>Any help is appreciated!</p>