title: Legend in python without labels
body: <p>I would like to show a fixed legend in a python matplotlib plot. I am creating a large set of plots, and some of them lack one of the datasets I am using. Let's say I have</p>
  
  <p>data1, plotted in green</p>
  
  <p>data2, plotted in blue </p>
  
  <p>data3, plotted in blue </p>
  
  <p>for some cases, dataX is missing, but I would like to show the whole legend (always the three colours). All I have found so far is based on labeling the data you plot, so: is there anyway to create a legend independent of the data you are plotting, and specify the symbol and colors when creating the legend?</p>
  
  <p>Thanks!</p>