title: R plotly link subplots so that multiple tooltips shown on hover
body: <p>I have data with groups. For every group I plot a subplot in plotly. The x-values for all of these subplots correspond to identical description, just the y-axis value is either missing or different across groups. I would like to link these subplots so that if I hover over one point in one subplot, it will also highlight the points with the same x value in other subplots. I have tried the crosstalk package examples, but either I create the subplots wrong or I don't understand how to use it. 
  Does anyone have any experience?</p>
  
  <p>Update:</p>
  
  <pre><code>df &lt;- data.frame(
       ID = rep(1:4, times = 2), 
       Value = rnorm(8),
       group = c(1,1,1,1,2,2,2,2)
   )
  
  subplot(
       plot_ly(df[df$group==1,], x = ~ID, y = ~Value),
       plot_ly(df[df$group==2,], x = ~ID, y = ~Value), nrows = 2
   )
  </code></pre>
  
  <p>And my question is, how could I highlight the points with the same x value in all subplots if I hover on one of the points?</p>