title: Bar heights and widths in histogram plot of several data
body: <p>I'm trying to plot a simple histogram with multiple data in parallel.<br>
  My data are a set of 2D <strong>ndarrays</strong>, all of them with the same dimension (in this example 256 x 256).</p>
  
  <p>I have this method to plot the data set:</p>
  
  <pre><code>def plot_data_histograms(data, bins, color, label, file_path):
          """
          Plot multiple data histograms in parallel
          :param data : a set of data to be plotted
          :param bins : the number of bins to be used
          :param color : teh color of each data in the set
          :param label : the label of each color in the set
          :param file_path : the path where the output will be save
          """
          plt.figure()
          plt.hist(data, bins, normed=1, color=color, label=label, alpha=0.75)
          plt.legend(loc='upper right')
          plt.savefig(file_path + '.png')
          plt.close() 
  </code></pre>
  
  <p>And I'm passing my data as follows:</p>
  
  <pre><code>data = [sobel.flatten(), prewitt.flatten(), roberts.flatten(), scharr.flatten()]
  labels = ['Sobel', 'Prewitt', 'Roberts Cross', 'Scharr']
  colors = ['green', 'blue', 'yellow', 'red']
  
  plot_data_histograms(data, 5, colors, labels, '../Visualizations/StatisticalMeasures/RMSEHistograms')
  </code></pre>
  
  <p>And I got this histogram:</p>
  
  <p><img src="https://i.stack.imgur.com/uzlpg.png" alt="histogram"></p>
  
  <p>I know that this may be stupid, but I didn't get why my <strong>yticks</strong> varies from 0 to 4.5. I know that is due the <strong>normed</strong> parameter, but even reading this;</p>
  
  <blockquote>
    <p>If <code>True</code>, the first element of the return tuple will be the counts
    normalized to form a probability density, i.e., <code>n/(len(x)*dbin)</code>. In a
    probability density, the integral of the histogram should be <code>1</code>; you
    can verify that with a trapezoidal integration of the probability
    density function.</p>
  </blockquote>
  
  <p>I didn't really get how it works.</p>
  
  <p>Also, once I set my <strong>bins</strong> to be equal five and the histogram has exactly 5 <strong>xticks</strong> (excluding borders), I didn't understand why I have some bars in the middle of some thicks, like the yellow one over the 0.6 thick. Since my number of <strong>bins</strong> and of <strong>xticks</strong> matches, I though that each set of four bars should be concentrated inside each interval, like it happens with the four first bars, completely concentrated inside the [0.0, 0.2] interval.</p>
  
  <p>Thank you in advance.</p>