title: Indexes in string - plotting graph
body: <p>How to write indexes in axes when I plot a graph? 
  I tried this, however, indexes are not print properly.
      import matplotlib.pyplot as plt
      plt.xlabel(r'$K$_1 (km s^{-1}'))</p>
  
  <p><img src="https://i.stack.imgur.com/T5z3J.png" alt="picture"></p>