title: how to use 'extent' in matplotlib.pyplot.imshow
body: <p>I managed to plot my data and would like to add a background image (map) to it.
  Data is plotted by the long/lat values and I have the long/lat values for the image's three corners (top left, top right and bottom left) too.</p>
  
  <p>I am trying to figure out how to use 'extent' option with imshow. However, the examples I found don't explain how to assign x and y for each corner ( in my case I have the information for three corners).</p>
  
  <p>How can I assign the location of three corners for the image when adding it to the plot?</p>
  
  <p>Thanks</p>