title: using add_tooltip in ggvis to print name when mouse hovers
body: <p>My data looks something like this:</p>
  
  <pre><code>df = data.frame(name=c("A1", "A2"),
                  x = c(2,4),
                  y = c(2,5),
                  sector = c("blue", "red"))
  </code></pre>
  
  <p>I am trying to use ggvis to create a graph but I am not able to make the tooltip work.</p>
  
  <pre><code>library(ggvis)
  df %&gt;% 
    ggvis(~x, ~y, size := 100, opacity := 0.4) %&gt;% 
    layer_points(fill = ~sector) %&gt;%
    add_tooltip(function(df) df$name)
  </code></pre>
  
  <p>When I hover the mouse <code>df$name</code> does not appear. What am I doing wrong?</p>
  
  <p>Thanks!</p>