title: Plotting netCDF with xarray, data not showing but legend is
body: <p>I am trying to use the simple .plot() function from xarray on a netCDF file from the <a href="http://fizz.phys.dal.ca/~atmos/martin/?page_id=140#V4.NA.02" rel="nofollow noreferrer">atmospheric composition analysis group</a>.</p>
  
  <p>Say I want to plot the PM2.5 concentration from North America in 2000 available <a href="http://fizz.phys.dal.ca/~atmos/martin/?page_id=140" rel="nofollow noreferrer">here</a>.</p>
  
  <p>When I try to plot the dataset I get an empty figure even though the data exists as shown by the legend bar.</p>
  
  <pre><code>import xarray as xr
  import netCDF4 as nc
  import matplotlib.pyplot as plt
  
  path_to_nc="my/path/file.nc"
  
  ds=xr.open_dataset(path_to_nc)
  print(ds)
  &gt;&gt;&gt;
  
  &lt;xarray.Dataset&gt;
  Dimensions:  (LAT: 4550, LON: 9300)
  Coordinates:
    * LON      (LON) float64 -138.0 -138.0 -138.0 -138.0 ... -45.03 -45.01 -45.01
    * LAT      (LAT) float64 68.0 67.99 67.97 67.96 ... 22.53 22.52 22.51 22.5
  Data variables:
      PM25     (LAT, LON) float32 ...
  </code></pre>
  
  <p>The file does have values (not just nan).</p>
  
  <pre><code># Range of values:
  ds=ds['PM25']
  print(ds)
  &gt;&gt;&gt;
  
  &lt;xarray.DataArray 'PM25' (LAT: 4550, LON: 9300)&gt;
  array([[1.6, 1.6, 1.6, ..., 1.2, 1.2, 1.2],
         [1.6, 1.6, 1.6, ..., 1.2, 1.2, 1.2],
         [1.6, 1.6, 1.6, ..., 1.2, 1.2, 1.2],
         ...,
         [nan, nan, nan, ..., nan, nan, nan],
         [nan, nan, nan, ..., nan, nan, nan],
         [nan, nan, nan, ..., nan, nan, nan]], dtype=float32)
  Coordinates:
    * LON      (LON) float64 -138.0 -138.0 -138.0 -138.0 ... -45.03 -45.01 -45.01
    * LAT      (LAT) float64 68.0 67.99 67.97 67.96 ... 22.53 22.52 22.51 22.5
  Attributes:
      standard_name:  PM25
      units:          ug/m3
  </code></pre>
  
  <p>But if I try to plot the values then I get an empty ax.</p>
  
  <pre><code>ds.plot()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/QSUSw.png" rel="nofollow noreferrer">Output</a></p>