title: plotly.py: change line opacity, leave markers opaque
body: <p>Is it possible to change the line opacity but not the marker opacity?
  I found that I can set the opacity of the entire line including markers (<code>opacity = .5</code>) and the one of the marker (e.g. <code>marker={"opacity":1}</code>).</p>
  
  <p>As shown in this example:</p>
  
  <pre><code>import plotly
  import plotly.graph_objs as go
  plotly.offline.init_notebook_mode(connected=True)  # I'm running in a jupyter notebook
  
  x = np.arange(0,10)
  ys = [np.random.rand(10) for _ in range(3)]
  
  lines = []
  for y in ys:
      line = go.Scatter(x=x, y=y, mode="markers+lines", opacity=.5, marker={'symbol': 'x', 'size': "15", "opacity":1})
      lines.append(line)           
  fig = go.Figure(
      data=lines,
      layout=go.Layout(showlegend=True)
  )
  plotly.offline.iplot(fig)
  </code></pre>
  
  <p>See result here:
  <a href="https://i.stack.imgur.com/8njnK.png" rel="noreferrer"><img src="https://i.stack.imgur.com/8njnK.png" alt="current result"></a></p>
  
  <p>My problem is the following: My data points are important, the lines are just visual aid. I want to make the lines .5-opaque but have the markers fully opaque.<br>
  However, when I set <code>opacity=.5, marker={'opacity':1}</code> the opacity of the marker is also reduced. (I believe that the marker-opacity is defined in the range <code>[0, line-opacity]</code>.</p>
  
  <p>Is there any way I can get the colour of the line and adjust its opacity (perhaps even after creating the line, but before plotting it).
  I know that I could create two traces, one with the points and one with the lines. However, I would like them to be the same colour without having to manually specify the colours. (The number of traces is varying, so I prefer sticking to the standard mechanism that assigns the different colours)</p>
  
  <p><strong>EDIT</strong>: My current solution is to set the line-width to 0.5 so it <em>looks</em> better, but obviously this solution works for me and might not be useful to people who want bold and less opaque lines.</p>
  
  <p><strong>EDIT</strong>: Github issue concerning this problem/feature request/behaviour:
  <a href="https://github.com/plotly/plotly.js/issues/2684" rel="noreferrer">https://github.com/plotly/plotly.js/issues/2684</a></p>