title: python Xarray DataArray: how do you add an additional coordinate to an existing dimension
body: <p>I am just starting to use <code>Xarray DataArray</code> and I was not sure <strong>how to add an new coordinate axis to an existing dimension</strong>. Can anyone point me in the correct direction. </p>
  
  <p>So I have an existing array that looks like:</p>
  
  <pre><code> demo_results = np.zeros([10, 20, 5])
   cols = ['c1', 'c2', 'c3', 'c4', 'c5']
   t = xr.DataArray(demo_results, coords={'sim': cols}, dims=('run', 'year', 'sim'))
  </code></pre>
  
  <p>So this will give me a 3 dimensional array with 5 coordinates along the 'sim' dimension. Now I wanted to add an additional coordinate on the 'sim' dimension named 'c6', but I was not sure how to do that? I did not see any info or examples in the documentation. In <code>pandas</code> I would just reference the new dimension like <code>df.loc[:, 'c6] = 0</code> and that would work. But I tried something like that with <code>Xarray</code> such as: </p>
  
  <pre><code> t.loc['sim', 'c6'] = 0 
  </code></pre>
  
  <p>but I go an error: <code>TypeError: invalid indexer array, does not have integer dtype: array('sim', dtype='&lt;U3')</code></p>
  
  <p>I guess I am still getting the hang of DataArray indexing. </p>