title: ggvis add_tooltip is not working with layer_bars
body: <p>I have the following code that I am trying to build into the shiny dashboard. If I change to layer_point the code works.</p>
  <p>I tried to change to layer_rect instead - however, I am unable to get a stacked bar chart with layer rects.</p>
  <p>I have attached a photo below error.</p>
  <p><a href="https://i.stack.imgur.com/QSUeH.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QSUeH.jpg" alt="enter image description here" /></a></p>
  <pre><code>color_choices &lt;- c(&quot;#08519C&quot;,&quot;#238B45&quot;)
  
  tab2_1 %&gt;%  
    ungroup() %&gt;%
    ggvis(x=~variable, y=~value, fill = ~Tender_won) %&gt;%
    layer_bars() %&gt;% 
    add_axis(&quot;y&quot;,title=&quot;Volume&quot;,title_offset=80) %&gt;%
    add_axis(&quot;x&quot;,title =&quot;Year&quot;) %&gt;%
    scale_ordinal(&quot;fill&quot;,range = color_choices) %&gt;%
    add_tooltip(function(data) paste0(&quot;test1&quot;,data$value,&quot;test2&quot;)) %&gt;%
    add_legend(scales = &quot;fill&quot;, values = c(&quot;No&quot;,&quot;Yes&quot;), title = &quot;Tender Status&quot;)
  </code></pre>
  <p>The Data can be found here:</p>
  <p><a href="https://www.dropbox.com/s/3lq6qncru0xuvoc/helphelp.csv?dl=0" rel="nofollow noreferrer">https://www.dropbox.com/s/3lq6qncru0xuvoc/helphelp.csv?dl=0</a></p>