title: Plotting a graph using matplotlib with two lists
body: <p>I am trying to plot a graph after having imported data from a CSV file and stored it as two separate lists. Is it possible for matplotlib to plot a graph using a list of strings, or is it necessary for the lists two be lists of "int" ? 
  If not, why is the following code not working?</p>
  
  <p>Error prompted:</p>
  
  <blockquote>
    <p>invalid literal for int() with base 10: '02_13_2014'</p>
  </blockquote>
  
  <p>My code:</p>
  
  <pre><code> import csv
      import numpy as np
      from numpy import genfromtxt
      import matplotlib.pyplot as plt
      import matplotlib.dates as mdates
  
  with open('pytdef.csv','r') as f:
      reader = csv.reader(f)
      for row in reader:
          print (row)
  
  first_column= ['Date']
  second_column = ['Value']
  Date_list = []
  Value_list = []
  
  with open('pytdef.csv') as f:
      reader = csv.reader(f)
      for row in reader:
          Date_list.append(row[0])
  
  with open('pytdef.csv') as f:
      reader = csv.reader(f)
      for row in reader:
          Value_list.append(row[1])
  
      print (Date_list)
      print (Value_list)
  
  Date_list = list(map(int,Date_list))
  print (Date_list)
  print (Value_list)
  
  
  fig = plt.figure()
  plt.plot_date(x=Date_list, y=Value_list)
  plt.show()
  </code></pre>