title: Sort 2D NumPy array by one of the columns
body: <p>I though this would be super easy but I am struggling a little. I have a data structure as follows</p>
  
  <pre><code>array([[  5.        ,   3.40166205],
     [ 10.        ,   2.72778882],
     [ 15.        ,   2.31881804],
     [ 20.        ,   2.50643777],
     [  1.        ,   3.94076063],
     [  2.        ,   3.80598599],
     [  3.        ,   3.67121134],
     [  6.        ,   3.2668874 ],
     [  7.        ,   3.13211276],
     [  8.        ,   2.99733811],
     [  9.        ,   2.86256347],
     [ 11.        ,   2.64599467],
     [ 12.        ,   2.56420051],
     [ 13.        ,   2.48240635],
     [ 14.        ,   2.4006122 ],
     [ 16.        ,   1.8280531 ],
     [ 17.        ,   1.74625894],
     [ 18.        ,   1.66446479],
     [ 19.        ,   1.58267063],
     [ 20.        ,   1.50087647]])
  </code></pre>
  
  <p>And I want to sort it ONLY on the first column ... so it is ordered as follows:</p>
  
  <pre><code>array([[1.  ,  3.9], 
         [2.  ,  3.8], 
         ...  , 
         [20. ,  1.5]])
  </code></pre>
  
  <p><code>np.sort</code> doesn't seem to work as it moves array to a flat structure. I've also used <code>itemgetter</code></p>
  
  <pre><code>from operator import itemgetter
  sorted(data, key=itemgetter(1))
  </code></pre>
  
  <p>But this doesn't give me the output I'm looking for.</p>
  
  <p>Help appreciated!</p>