title: nvd3 piechart.js - How to edit the tooltip?
body: <p>I'm using nvd3's piechart.js component to generate a piechart on my site. The provided .js file includes several var's, as follows:</p>
  
  <pre><code>var margin = {top: 30, right: 20, bottom: 20, left: 20}
      , width = null
      , height = null
      , showLegend = true
      , color = nv.utils.defaultColor()
      , tooltips = true
      , tooltip = function(key, y, e, graph) {
          return '&lt;h3&gt;' + key + '&lt;/h3&gt;' +
                 '&lt;p&gt;' +  y + '&lt;/p&gt;'
        }
      , noData = "No Data Available."
      , dispatch = d3.dispatch('tooltipShow', 'tooltipHide')
  ;
  </code></pre>
  
  <p>In my in-line js, I've been able to override some of those variables, like this (overriding showLegend and margin):</p>
  
  <pre><code>var chart = nv.models.pieChart()
      .x(function(d) { return d.label })
      .y(function(d) { return d.value })
      .showLabels(false)
      .showLegend(false)
      .margin({top: 10, right: 0, bottom: 0, left: 0})
      .donut(true);
  </code></pre>
  
  <p>I've tried overwriting the tooltip in the same way:</p>
  
  <pre><code>.tooltip(function(key, y, e, graph) { return 'Some String' })
  </code></pre>
  
  <p>but when I do that, my piechart does not display at all. Why can't I overwrite the tooltip here? Is there another way I can do so? I'd really rather not have to edit piechart.js itself at all; I need to keep that file generic for use in multiple widgets.</p>
  
  <p>And while we're at it, is there some way I can make the entire tooltip into a clickable link?</p>