title: pcolormesh shading='gouraud' not working
body: <p>I am trying to smooth my color map using pcolormesh shading='gouraud' argument, but it failed, returned the follow error, which I do not understand. </p>
  
  <pre><code>"/usr/local/anaconda/lib/python2.7/site- packages/matplotlib/collections.py", line 1822, in draw
  gc, triangles, colors, transform.frozen())
  File "/usr/local/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_ps.py", line 876, in draw_gouraud_triangles
  ('colors', 'u1', (3,))])
  TypeError: data type not understood  File
  </code></pre>
  
  <p>I have my code as below:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  from matplotlib import rcParams
  from matplotlib.mlab import griddata
  from matplotlib.ticker import AutoMinorLocator
  from mpl_toolkits.axes_grid1 import make_axes_locatable
  from mpl_toolkits.basemap import Basemap
  
  x = np.loadtxt('data.txt',usecols=[0])
  y = np.loadtxt('data.txt',usecols=[1])
  s = np.loadtxt('data.txt',usecols=[2])
  
  N = 36j
  M = 18j
  extent = (min(x), max(x), min(y), max(y))
  xx,yy = np.mgrid[extent[0]:extent[1]:N, extent[2]:extent[3]:M]
  ss = griddata(x, y, s, xx, yy, interp='linear')
  
  fig, ax = plt.subplots()
  #m = Basemap(projection='hammer',lon_0=0,resolution=None)
  m = Basemap(projection='kav7',lon_0=0,resolution=None)
  m.drawmapboundary(fill_color='0.75')
  im = m.pcolormesh(xx,yy,ss,shading='gouraud',cmap=plt.cm.jet,latlon=True)
  m.drawparallels(np.arange(-90.,99.,30.))
  m.drawmeridians(np.arange(-180.,180.,60.))
  cb = m.colorbar(im,"bottom", size="5%", pad="2%", ticks=[-2,-1,0,1,2,3,4,5])
  plt.show()
  </code></pre>
  
  <p>When the shading argument is flat <code>shading = 'flat'</code>, then this works very well, but the color is not smooth. Any one can offer me some idea how to approach this problem?  </p>