title: How to plot density of lines with plotly
body: <p>Let's say I have a set lines (y vs. x) that are the result of a random process.</p>
  
  <p>I am looking for a visualization that shows the spread of the lines.</p>
  
  <p>Concretely I am looking to get an e.g. transparency-graded filled region where the opacity corresponds to the height of the histogram at that point.</p>
  
  <p>Somewhat like the <a href="https://plot.ly/python/line-charts/#filled-lines" rel="nofollow noreferrer">plotly "Filled Lines" example</a>, except that I am not looking for hard borders to the area filled but a gradual phasing out using transparency.</p>
  
  <p>One example similar to what I am looking to realize is shown in on <a href="http://blog.enthought.com/general/visualizing-uncertainty/" rel="nofollow noreferrer">this blog</a>:
  <a href="https://i.stack.imgur.com/eBnnE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eBnnE.png" alt="enter image description here"></a></p>
  
  <p>I would like to plot different sets of lines (e.g. different experimental conditons) on the same plot in different colours to visually compare their results.</p>