title: Bold format with mathematical expression in matplotlib
body: <p>I have problems in using tex expression into matplotlib and putting them into bold format. I cannot at the moment find a solution.</p>
  
  <p>Here is a minimal example. How can I put the subscripts, superscripts and the greek-letters into bold format ?</p>
  
  <hr>
  
  <pre><code>import matplotlib.pyplot as plt 
  import numpy as np
  from matplotlib import rc
  
  rc('text', usetex=True)
  
  x = np.arange(10)
  
  first_plot,  = plt.plot(x[:],2*x[:],'k-')
  
  second_plot, = plt.plot(x[:],4*x[:],'k-')
  
  third_plot,  = plt.plot(x[:],6*x[:],'k-')
  
  plt.legend([first_plot,second_plot,third_plot],[r'\textbf{A}$_\sigma$',
  r'\textbf{B}',r'\textbf{a$^2$}'], fontsize=36, loc=[0.4,0.70],frameon=False)
  
  plt.show()
  </code></pre>