title: R shiny ggvis tooltip will not update information with new data, even though plot updates
body: <p>So I have a ggvis plot within a shiny application, which is a histogram that that displays the count of observations in a bin when the user hovers over the bin. The data is randomly generated and the user can hit a button to sample a new batch of data. Everything works fine except that after a new batch of data is sampled and plotted, the tooltip still displays the counts in each bin from the original data, which of course does not match with the current data. Is there a way to fix this? Below is my minimal reproducible example.</p>
  
  <pre><code>toolTipFunc &lt;- function(value) {
    function(y) {
      if (is.null(y)) return(NULL)
      else {
        length(value[(value &gt;= y$xmin_) &amp; (value &lt; y$xmax_)])
      }
    }
  }
  
  
  library(shiny)
  library(ggvis)
  
  runApp(list(
  
  ui = pageWithSidebar(
  div(),
  sidebarPanel(
     actionButton("sampleButton", "Sample")),
  
   mainPanel(
     ggvisOutput("plot"))),
  
   server = function(input, output) {
  
     sampleInput &lt;- reactive({
       input$sampleButton
       x &lt;- rnorm(500,0,1.5)
       data.frame(x)
     })
  
     gv &lt;- reactive({
       df &lt;- sampleInput()
       df %&gt;% ggvis(~x) %&gt;% 
         add_tooltip(toolTipFunc(df$x), "hover") %&gt;% 
         layer_histograms(width=0.5) %&gt;%
         set_options(width=540,height=350,keep_aspect=TRUE)
     })
  
     gv %&gt;% bind_shiny("plot")
  
   }))
  </code></pre>