title: angular nvd3 xAxisTickFormatFunction updating the data which I do not want
body: <p>I have two functions been used in the angular nvd3. They are xAxisTickFormatFunction and toolTipContentFunction.</p>
  
  <pre><code> $scope.xAxisTickFormatFunction = function() {
     return function(d) {
            return d3.time.format("%b, %Y")(new Date(d));
          };
    }
  
    $scope.toolTipContentFunction = function() {
      return function(d) {
        return  d3.time.format("%Y-%m-%d")(new Date(d));
      }
    }
  </code></pre>
  
  <p>which d in both function refers to the same value. The original value of d is a date type like "2014-06-20", but after been used in xAxisTickFormatFunction, it turned out to be "June, 2014", and then in the toolTipContentFunction, it's value will be showed as "2014-06-01" instead of "2014-06-20", which implied that the data is missing because of the use of formatting.
  How can I avoid the data been updated?</p>