title: "Invalid slice" when ploting using matplotlib
body: <p>Python 2.7.5 and OSX 10.8 here</p>
  
  <p>I'm trying to plot some datas I get from a file.
  I'm trying to code a function where the user is able to plot from the point he wants, and have come to this part of code :  </p>
  
  <pre><code>firstPoint = raw_input("1st point to be displayed is n° : ")
  
  tbdata3 = hdulist[3].data
  print(hdulist[3].header['TTYPE24'])
  print tbdata3.field('DDL_FT_OPL')
  print(hdulist[3].header['TTYPE23'])
  print tbdata3.field('DDL_SC_OPL')
  
  dataFT=tbdata3.field('DDL_FT_OPL')
  plt.subplot(211)
  plot(dataFT[firstPoint:400,:])
  dataSC=tbdata3.field('DDL_SC_OPL')
  plt.subplot(212)
  plot(dataSC[firstPoint:400,:])
  show()
  </code></pre>
  
  <p>I get this error :</p>
  
  <pre><code>Traceback (most recent call last):
  File "/Users/geoffroysarrazin/Desktop/stage_observatoire/testEkki.py", line 52, in essai
  plot(dataFT[firstPoint:400,:])
  IndexError: invalid slice
  </code></pre>
  
  <p>And it seems weird to me, cause I got this with </p>
  
  <pre><code>firstPoint=10
  </code></pre>
  
  <p>and just before, I had a constant value instead of this input from the user, which was equal to 200 (or whatever &lt;400) and it worked...</p>