title: Plotly: Change transparency of fillcolor
body: <p>I'm trying to change the default color of a fillcolor in Plotly to another transparent one, but when I change the fillcolor it is not transparent anymore. </p>
  
  <pre><code>trace = (go.Scatter(x=[30,45],y=[3000,3000],
                  fill='tozeroy', 
  #                     fillcolor='green',
  #                     fillcolor='rgb(26,150,65,0.5)',
  #                     fillcolor=dict(color='rgb(26,150,65,0.5)'),
  #              fillcolor=dict(marker=dict(color='rgb(26,150,65,0.5)')),
                  opacity=0.1,
                  showlegend=False,
                  hoverinfo='none',
                  mode='none'))
  py.offline.iplot(go.Figure(data=[trace]))
  </code></pre>
  
  <p>This <em>default</em> call results in the following:
  <a href="https://i.stack.imgur.com/Q65q6.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Q65q6.png" alt="default fillcolor"></a></p>
  
  <p>and when I use <code>fillcolor='green'</code> it results in this nontransparent color:
  <a href="https://i.stack.imgur.com/UWyd4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/UWyd4.png" alt="fillcolor = green"></a></p>
  
  <p>The commented code results in the default case again.</p>