title: Pcolormesh on basemap
body: <p>I have a long list of latitudes and longitudes, and the ultimate goal is to plot the number of occurrences at each latitude/longitude pair on a map of North America.  As an example, I have a DataFrame, called sort_df, that looks like:</p>
  
  <pre><code>longitude  -95  -94  -93  -92
  latitude
  43           0    4    8    3
  44           7    3    0    0 
  </code></pre>
  
  <p>using sort_df.values and pmeshcolor, I can plot the data.  The code is as follows:</p>
  
  <pre><code>m.Basemap(llcrnrlon = -170, llcrnrlat = 10, urcrnrlon = -50, urcrnrlat = 80)
  m.drawcoastlines()
  m.drawstates()
  ny = sort_df.shape[0]
  nx = sort_df.shape[1]
  lons, lats = m.makegrid(nx,ny)
  x,y = m(lons,lats)
  my_cmap = plt.get_cmap('rainbow')
  my_cmap.set_under('white')
  data = np.array(sort_df.values)
  cs = m.pcolormesh(x,y,data,cmap = my_cmap, vmin = 1)
  m.colorbar(cs,extend = 'min')
  plt.show()
  </code></pre>
  
  <p>The above code produces a plot that looks like:
  <a href="https://i.stack.imgur.com/jxuiv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jxuiv.png" alt="enter image description here"></a></p>
  
  <p>The problem: There should only be colors on the lat/lon grid defined by sort_df.</p>
  
  <p>Question: How do I make the grid appear in the correct spots on the map?     </p>