title: Adding points to an existing scatter plot with plotly
body: <p>Suppose I have the following data frames : </p>
  
  <pre><code>a &lt;- c(1,2,2,3,7,9,8,3,7,9)
  b &lt;- c(1:10)
  df1 &lt;- data.frame(a,b)
  c &lt;- c(2,5,4)
  d &lt;- c(3,6,5)
  df2 &lt;- data.frame(c,d)
  </code></pre>
  
  <p>And I generate a scatter plot of df1 with plotly :</p>
  
  <pre><code>p &lt;- plot_ly(data=df1, x=a, y=b)
  </code></pre>
  
  <p>Then how can I add points from df2 to p with, for instance, a different color?</p>