title: python: plotly bar graph using y0 and dy offset
body: <p>I'm investigating using <code>plotly</code> to replace some graphing I currently do in <code>matplotlib</code>.</p>
  
  <p>I need to control a bar graph to have the bottom and top of each bar start and end at arbitrary vlaues.</p>
  
  <p>In <code>matplotlib</code> this is fairly straight forward:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  
  plt.bar(0.5, height=4,bottom=2)
  plt.ylim(0, 8)
  plt.xlim(0, 2)
  
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/UiHgd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UiHgd.png" alt="enter image description here"></a></p>
  
  <p>The <a href="https://plot.ly/python/reference/#bar-y0" rel="nofollow noreferrer">docs for plotly</a> seem to indicate I can do something similar by using <code>y0</code> and <code>dy</code> arguments in the <code>Bar</code> object like so:</p>
  
  <pre><code>import plotly.offline as py
  import plotly.graph_objs as go
  
  trace1 = go.Bar(
      x=['x'],
      y0=2,
      dy=6,
  )
  
  fig = go.Figure(data=[trace1])
  py.iplot(fig)
  </code></pre>
  
  <p>But this just gives me an empty plot?!</p>
  
  <p><a href="https://i.stack.imgur.com/Ew90q.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ew90q.png" alt="enter image description here"></a></p>
  
  <p>I also tried feeding the above matplotlib object into plotly's <code>py.plot_mpl()</code> but this raises <code>PlotlyEmptyDataError</code>.</p>
  
  <p>thanks
  JP</p>