title: matplotlib: Convert Hour string to float
body: <p>I'm trying to plot the following csv data as the x-axis with matplotlib:</p>
  
  <pre><code>Time
  00:00:00
  01:00:00
  02:00:00
  03:00:00
  04:00:00
  05:00:00
  06:00:00
  07:00:00
  08:00:00
  09:00:00
  10:00:00
  11:00:00
  12:00:00
  13:00:00
  14:00:00
  15:00:00
  16:00:00
  17:00:00
  18:00:00
  19:00:00
  20:00:00
  21:00:00
  22:00:00
  23:00:00
  </code></pre>
  
  <p>But I keep getting the error:</p>
  
  <pre><code>ValueError: could not convert string to float: '23:00:00'
  </code></pre>
  
  <p>How Can I convert these values to Hour format in matplotlib?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>fig = plt.figure(figsize=(15,7))
  ax = fig.add_subplot(1,1,1) # row-col-num
  # --- line plot data on the Axes
  ax.plot(df3['YYYY-MO-DD HH-MI-SS_SSS'], df3['ATMOSPHERIC PRESSURE (hPa) mean'], 'b-', linewidth=2,
  label=r'office Phone1')
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>Thank you !</p>