title: matplotlib overlay a normal distribution with stddev axis onto another plot
body: <p>I have a series of data that I'm reading in from a tutorial site.
  I've managed to plot the distribution of the TV column in that data, however I also want to overlay a normal distribution curve with StdDev ticks on a second x-axis (so I can compare the two curves). I'm struggling to work out how to do it..</p>
  
  <pre><code>import pandas as pd
  import matplotlib
  import matplotlib.pyplot as plt
  import numpy as np
  import scipy.stats as stats
  import matplotlib.mlab as mlab
  import math
  
  # read data into a DataFrame
  data = pd.read_csv('http://www-bcf.usc.edu/~gareth/ISL/Advertising.csv', index_col=0)
  
  # draw distribution curve
  h = sorted(data.TV)
  hmean = np.mean(h)
  hstd = np.std(h)
  pdf = stats.norm.pdf(h, hmean, hstd)
  plt.plot(h, pdf)
  </code></pre>
  
  <p>Here is a diagram close to what I'm after, where x is the StdDeviations. All this example needs is a second x axis to show the values of data.TV</p>
  
  <p><img src="https://onlinecourses.science.psu.edu/stat464/sites/onlinecourses.science.psu.edu.stat464/files/lesson02/plot_normal_cauchy.gif" alt="example image"></p>