title: How can I show a km ruler on a cartopy / matplotlib plot?
body: <p>How can I show a km ruler for a zoomed in section of a map, either inset in the image or as rulers on the side of the plot?</p>
  
  <p>E.g. something like the 50 km bar on the side (left) or the inset in mi (right): </p>
  
  <p><img src="https://i.stack.imgur.com/kJwH9.jpg" width="300">
  <img src="https://i.stack.imgur.com/CZTgD.jpg" width="300"></p>
  
  <p>(sources: <a href="http://se.mathworks.com/products/mapping/features.html" rel="noreferrer">1</a>, <a href="http://se.mathworks.com/products/mapping/features.html" rel="noreferrer">2</a>)</p>
  
  <p>(issue: <a href="https://github.com/SciTools/cartopy/issues/490" rel="noreferrer">cartopy#490</a>)</p>