title: matplotlib.pyplot.plot, ValueError: could not convert string to float: f
body: <p>I'm trying to use python to make plots. Below is the simplified version of my code     that cause error.</p>
  
  <pre><code>import numpy as np
  import matplotlib   
  import matplotlib.pyplot as plt
  matplotlib.use("AGG")
  
  distance = np.array('f')
  depth = np.array('f')# make sure these two arrays store float type value
  
  with open('Line671.txt','r') as datafile:
    for line in datafile:
      word_count = 0
      for word in line.split():
          word = float(word)#convert string type to float
          if word_count == 0:
              distance = np.append(distance, word)
          elif word_count == 1:
              depth = np.append(depth, word)
          else:
            print 'Error'
          word_count += 1
  
  datafile.closed
  
  
  print depth
  print distance #outputs looks correct 
  # original data is like this: -5.3458000e+00
  # output of the array is :['f' '-5.3458' '-5.3463' ..., '-5.4902' '-5.4912' '-5.4926']
  
  plt.plot(depth, distance)# here comes the problem  
  </code></pre>
  
  <p>The error message says that in line for plt.plot(depth, distance): ValueError: could not convert string to float: f<br>
  I don't understand this because it seems I converted all string values into float type. I tried to search this problem on stackoverflow but they all seem to solve the problem once they cast all string values into float or int. Can anyone give any suggestion on this problem? I would be really appreciate for any help.</p>