title: Matplotlib subscript in format string
body: <p>I know that this question has been asked before <a href="https://stackoverflow.com/questions/22322682/matplotlib-subscript">here</a>, but the answer given does not work for me.. 
  When I put {} around my subscript, I am getting a Key Error. It happens whether I use "G$<em>{LA}$" or "G</em>{LA}".</p>
  
  <p>As an edit, this is a simplified version of the code I'm using:</p>
  
  <pre><code>X=np.array(random.sample(range(1000),10))
  Y=np.array(random.sample(range(1000),10))
  
  plt.clf()
  f, ax =plt.subplots(1,1)
  
  b, a = np.polyfit(X,Y, 1)
  
  # 3.5 Calculate Pearson's correlation
  r, SE = stats.pearsonr(X, Y)
  r2 = r*r
  
  # 3.6 Regression line to be added to plot - anchored text automatically locates the text, without the need to specifying the x,y coordinates
  anchored_text = AnchoredText("GLA = {:3.2f}*GLD + {:3.2f}\n\nPearson's R = {:3.2f}, SE = {:3.2f}\n $R^2$ = {:3.2f}".format(b,a,r,SE,r2), loc=2)
  
  # 3.7 Plot
  ax = sns.regplot(X, Y, color='g')
  ax.add_artist(anchored_text)
  
  plt.show()
  </code></pre>
  
  <p>Now, if I add the subscript, it throws in the error:</p>
  
  <pre><code>---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  &lt;ipython-input-31-00e10c71309f&gt; in &lt;module&gt;()
       19 
       20 # 3.6 Regression line to be added to plot - anchored text automatically locates the text, without the need to specifying the x,y coordinates
  ---&gt; 21 anchored_text = AnchoredText(r"G$_{LA}$ = {:3.2f}*GLD + {:3.2f}\n\nPearson's R = {:3.2f}, SE = {:3.2f}\n $R^2$ = {:3.2f}".format(b,a,r,SE, r2), loc=2)
       22 
       23 # 3.7 Plot
  
  KeyError: 'LA'
  </code></pre>
  
  <p>Would that be because I am using AnchoredText? Or has anything changed since the other thread was posted?</p>