Illustrates issues with matplotlib contouring in IPython Notebook. Note, you can take the code in cell run and run it through the IPython console and it works just fine.

ipynb_contour_issue.ipynb

{
 "metadata": {
  "name": "Untitled0"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "code",
     "collapsed": false,
     "input": "import numpy as np\nimport matplotlib.pyplot as plt\n\nlength = 20\n\ngrid = np.ones((length, length), dtype=float)\nx = np.arange(length)\ny = np.arange(length)\ngrid[x, y] = 0.5\ngrid[x, y[::-1]] = 0.5\ngrid[x[::-1], y[::-1]] = 0.5\ngrid[x[::-1], y] = 0.5\n\nx = range(length)\ny = x\n\nplt.contourf(x, y, grid, levels=[0, 0.25, 0.5, 0.75, 1., 1.25])\n",
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 1,
       "text": "<matplotlib.contour.QuadContourSet instance at 0x106b48248>"
      },
      {
       "output_type": "display_data",
       "text": "<matplotlib.figure.Figure at 0x10671c510>"
      }
     ],
     "prompt_number": 1
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": "plt.pcolormesh(grid)",
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 2,
       "text": "<matplotlib.collections.QuadMesh at 0x106b5fed0>"
      },
      {
       "output_type": "display_data",
       "png": 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       "text": "<matplotlib.figure.Figure at 0x106b35050>"
      }
     ],
     "prompt_number": 2
    }
   ],
   "metadata": {}
  }
 ]
}