title: use Geopandas plot missing values
body: <p>My shapefile has some missing values (represented by <code>nan</code>) on certain columns (for example, GDP). When plotting without dealing with those missing values, the legend shows like this:
  <a href="https://i.stack.imgur.com/ObZOr.jpg" rel="noreferrer">enter image description here</a></p>
  
  <p>which is not what I want.
  So, I replace the missing values with a string "missing", then redo the plotting. Not surprisingly, I got the error message saying that <code>TypeError: '&lt;' not supported between instances of 'str' and 'float'</code>.</p>
  
  <p>My questions are: 1. how does Geopandas treat missing values? Does it store the missing values in a string or some other types of data? 2. How can I keep those missing values and redo the plotting with the legend label show the missingness?</p>