title: Pyplot - bar chart of positive and negative values
body: <p>I want the negative bars to be facing downwards, and the positive upwards, with the x-axis(0-line) passing right between them. I tried this</p>
  
  <pre><code>chart = fig.bar(x, negative_data, width=35, color='r')
  ax2 = plt.gca().twinx()
  ax2.bar(x, positive_data, width=35, color='b')
  </code></pre>
  
  <p>But instead, I get merged red and white bars, both facing downwards. It seems that the arrays negative_data/positive_data only specify the height of the bar, but how do I specify the orientation? I need something to specify the coordinates of the  tops of each bar.</p>
  
  <p>Also, how do I make the widths be something reasonable, possibly dynamic as the graph is resized by user?</p>
  
  <p>Here is an example of problematic widths:</p>
  
  <pre><code>x = [250, 1500, 2750, 4250, 6000, 8500, 13200]
  negative_data = [0, 0, 0, 0, 0, 0, 0]
  positive_data = [3, 0, 0, 0, 1, 0, 0]
  </code></pre>
  
  <p>How can I make the plot of those look nice?</p>