title: How can I update plotly subplots when changing traces?
body: <p>Here's an reproductive example which I desperately need to find a tricky solution for. Using Python and Plotly:</p>
  
  <pre><code>import plotly.plotly as py
  import plotly.graph_objs as go
  import numpy as np
  import plotly.offline as pyo
  
  # Many random traces
  d = []
  for i in range(0,10):
      d.append(go.Scatter({'y': np.random.randint(0,10,10), 'x': np.array(range(0,11))}))
  
  # One trace solving averages for above traces
  t = np.array([i['y'] for i in d])
  d2 = np.mean(a=t, axis=0)
  
  trace2 = go.Scatter(x = np.array(range(0,11)), y = d2, xaxis='x2', yaxis='y2', name='averages')
  
  data = d+[trace2]
  layout = go.Layout( 
      xaxis1={'anchor': 'y1'},
      yaxis1={'anchor': 'x1', 'domain': [0, .5]},
      xaxis2={'anchor': 'y2'},
      yaxis2={'anchor': 'x2', 'domain': [0.55, 1]}
  )
  fig = go.Figure(data=data, layout=layout)
  pyo.plot(fig)
  </code></pre>
  
  <p>The problem is when I click on legend's traces to hide the average trace from second subplot is not updating. How to do the trick? How to make it updating without button event - just on pure click on traces in legend? I spent all day yesterday to find a solution and I guess there's no besides triggering button event. But even with button I'd like to know which traces are still visible - it may lead to much more complicated plot than it should be.</p>
  
  <p>I'm going to use it in plotly-dash.</p>