title: fix plotly color scale breaks
body: <p>I have created a heatmap using plotly's R interface, like so:</p>
  
  <pre><code>df &lt;- data.frame(date = rep(seq(ymd('2016-09-01'), ymd('2016-09-10'), by='day'), each=2),
                   period = rep(c('lunch', 'dinner'), times=10),
                   diff=rnorm(20))
  
  df %&gt;%
    plot_ly(x = date,
            y = period,
            z = abs(diff),
            type = "heatmap",
            hoverinfo='none',
            colors = rev(RColorBrewer::brewer.pal(5, "RdYlGn")),
            showscale = F) %&gt;%
    add_trace(x = date,
              y = period,
              text = paste0(round(diff*100), '%'),
              mode='text',
              hoverinfo='none')
  </code></pre>
  
  <p>which produces this:</p>
  
  <p><a href="https://i.stack.imgur.com/ha91g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ha91g.png" alt="heatmap"></a></p>
  
  <p>Is there any way I can fix the breaks of the color scale? I would like the midpoint of the color scale to be at 20% and any value that exceeds 50% to be colored with the red at the end of the scale.</p>