title: Creating a definition to overlay or more plt.plot() using definitions in Python
body: <p>For the life of me I can't figure this out, and I've been at it for a few hours now.  I would like to create a chart, that I can overlay multiple charts on. I've created a definition for the chart, but it doesn't return anything (I've tried to modify it to return an ax, but that doesn't work either). As it stands it outputs two seperate charts, perfectly the way I want it to work.  However, I'd like it to print 1 chart, with both lines overprinted. Here is where my code is at:</p>
  <pre><code>def plot_tera_wasserburg_concordia(lambda238=1.54*10**-10,
                     lambda235=9.72*10**-10,
                     show_ages=True,
                     title=&quot;Tera-Wasserburg Concordia Diagram&quot;,
                     linecolor=&quot;r&quot;):
      df = get_ratio_data(lambda238, lambda235)   # I've defined this elsewhere 
      fig = plt.figure(figsize=(12,16))
      ax = fig.add_subplot()
      ax.spines['right'].set_color('none')
      ax.spines['top'].set_color('none')
      ax.set_xlim([0,35])
      ax.set_ylim([0,0.62])
      plt.xlabel(&quot;238U/206Pb&quot;, fontsize=16)
      plt.ylabel(&quot;207Pb/206Pb&quot;, fontsize=16)
      plt.title(title, fontsize=22)
      if show_ages == True:
          get_ages(lambda238, lambda235, 'tw') # I've defined this elsewhere
      plt.plot(df[&quot;ratio_238U_206Pb&quot;], df[&quot;ratio_207Pb_206Pb&quot;], linecolor, zorder=1)
      
  geochron.plot_tera_wasserburg_concordia(show_ages=False)
  
  lambda238=1.55125*10**-10
  lambda235=9.8485*10**-10
  
  geochron.plot_tera_wasserburg_concordia(lambda238,
                                          lambda235,
                                          show_ages=False,
                                          linecolor=&quot;g&quot;)
  
  plt.show()
  </code></pre>