title: D3.js Enter/update/exit not adding elements as expected
body: <p>I have a day/hour heatmap based on <a href="http://bl.ocks.org/tjdecke/5558084" rel="nofollow">this example</a>. I'm trying to write an updateHeatMap method so that I can dynamically update the heatmap with new data. </p>
  
  <p>I'm using the enter/update/exit methods as suggested in several tutorials and here is the behavior that I am noticing. I open the page and the data is there, great. I update the data and it has fewer elements, so the old elements disappear, this is exactly what I want. I update the data again with new elements, and they do not appear, this is not what I want.</p>
  
  <p>The initHeatMap function is only called once on load, the updateHeatMap function is called every time the data is updated.</p>
  
  <p>I have changed the original day/hour heatmap example slightly to look like this:</p>
  
  <pre><code>$scope.heatMapData = $http.get(...
  
  $scope.initHeatMap = function() {
  
  $scope.margin = { top: 50, right: 0, bottom: 100, left: 30 },
    width = 960 - margin.left - margin.right,
    height = 430 - margin.top - margin.bottom,
    gridSize = Math.floor(width / 24),
    legendElementWidth = gridSize*2,
    buckets = 9,
    colors = ["#ffffd9","#edf8b1","#c7e9b4","#7fcdbb","#41b6c4","#1d91c0","#225ea8","#253494","#081d58"], // alternatively colorbrewer.YlGnBu[9]
    days = ["Mo", "Tu", "We", "Th", "Fr", "Sa", "Su"],
    times = ["1a", "2a", "3a", "4a", "5a", "6a", "7a", "8a", "9a", "10a", "11a", "12a", "1p", "2p", "3p", "4p", "5p", "6p", "7p", "8p", "9p", "10p", "11p", "12p"];
  
  
  
    $scope.colorScale = d3.scale.quantile()
        .domain([0, buckets - 1, d3.max($scope.heatMapData, function (d) { return d.value; })])
        .range(colors);
  
    var svg = d3.select("#heatmap").append("svg")
        .attr("width", width + margin.left + margin.right)
        .attr("height", height + margin.top + margin.bottom)
        .append("g")
        .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
  
    var dayLabels = svg.selectAll(".dayLabel")
        .data(days)
        .enter().append("text")
          .text(function (d) { return d; })
          .attr("x", 0)
          .attr("y", function (d, i) { return i * gridSize; })
          .style("text-anchor", "end")
          .attr("transform", "translate(-6," + gridSize / 1.5 + ")")
          .attr("class", function (d, i) { return ((i &gt;= 0 &amp;&amp; i &lt;= 4) ? "dayLabel mono axis axis-workweek" : "dayLabel mono axis"); });
  
    var timeLabels = svg.selectAll(".timeLabel")
        .data(times)
        .enter().append("text")
          .text(function(d) { return d; })
          .attr("x", function(d, i) { return i * gridSize; })
          .attr("y", 0)
          .style("text-anchor", "middle")
          .attr("transform", "translate(" + gridSize / 2 + ", -6)")
          .attr("class", function(d, i) { return ((i &gt;= 7 &amp;&amp; i &lt;= 16) ? "timeLabel mono axis axis-worktime" : "timeLabel mono axis"); });
  
    var heatMap = svg.selectAll(".hour")
        .data($scope.heatMapData)
        .enter().append("rect")
        .attr("x", function(d) { return (d.hour - 1) * gridSize; })
        .attr("y", function(d) { return (d.day - 1) * gridSize; })
        .attr("rx", 4)
        .attr("ry", 4)
        .attr("class", "hour bordered")
        .attr("width", gridSize)
        .attr("height", gridSize)
        .style("fill", colors[0]);
  
    heatMap.transition().duration(1000)
        .style("fill", function(d) { return $scope.colorScale(d.value); });
  
    heatMap.append("title").text(function(d) { return d.value; });
  
    var legend = svg.selectAll(".legend")
        .data([0].concat($scope.colorScale.quantiles()), function(d) { return d; })
        .enter().append("g")
        .attr("class", "legend");
  
    legend.append("rect")
      .attr("x", function(d, i) { return legendElementWidth * i; })
      .attr("y", height)
      .attr("width", legendElementWidth)
      .attr("height", gridSize / 2)
      .style("fill", function(d, i) { return colors[i]; });
  
    legend.append("text")
      .attr("class", "mono")
      .text(function(d) { return "≥ " + Math.round(d); })
      .attr("x", function(d, i) { return legendElementWidth * i; })
      .attr("y", height + gridSize);
    };
  </code></pre>
  
  <p>Here is the update method:</p>
  
  <pre><code>$scope.rowSelected() = function(){
    $http.get(...).then(function(result){
      $scope.heatMapData = result.data;
      $scope.updateHeatMap();
    });
  }
  
  $scope.updateHeatMap = function(){
      var selection = d3.select("#heatmap");
      var heatMap = selection.selectAll(".hour").data($scope.heatMapData);
  
      heatMap.enter().append("rect")
        .attr("x", function(d) { return (d.hour - 1) * gridSize; })
        .attr("y", function(d) { return (d.day - 1) * gridSize; })
        .attr("rx", 4)
        .attr("ry", 4)
        .attr("class", "hour bordered")
        .attr("width", gridSize)
        .attr("height", gridSize)
        .style("fill", colors[0]);
  
     heatMap.transition().duration(1000)
            .style("fill", function(d) {return $scope.colorScale(d.value);});
  
     heatMap.exit().remove();
  };
  </code></pre>