title: How can I get tooltips showing in dygraphs without annotation
body: <p>I am trying to use the R implementation of dygraphs</p>
  
  <p>The example provided is</p>
  
  <pre><code>library(dygraphs)
  
  dygraph(presidents, main = "Presidential Approval") %&gt;%
  dyAxis("y", valueRange = c(0, 100)) %&gt;%
  dyAnnotation("1950-7-1", text = "A", tooltip = "Korea") %&gt;%
  dyAnnotation("1965-1-1", text = "B",  tooltip = "Vietnam")
  </code></pre>
  
  <p>which results in the chart<img src="https://i.stack.imgur.com/6iiYZ.png" alt="enter image description here"> </p>
  
  <p>Hovering over the 'A' produces a tooltip with 'Korea'</p>
  
  <p>I am keen to have a tooltip for every point preferably dispensing entirely with the text requirement - though setting text to "" with minimal height/width values might suffice. I would also want to attach the dyAnnotations programmatically from a file with date and tooltip columns</p>
  
  <pre><code>df &lt;- data.frame(date=as.Date(c("1950-7-1","1965-1-1")),tooltip=c("Korea","Vietnam"))
  </code></pre>
  
  <p>Is this feasible,and, if so, how?
  TIA</p>