                 .x d3.scale.linear()
                 .xAxisPadding padding[xAxis]
 
-            # Supply the tooltips and hide plot points where the X or Y value
-            # is null.
             chart.on 'renderlet', (chart) ->
               chart.selectAll '.symbol'
-                .on 'mouseover', (d) ->
+                # Supply the tooltips.
+                .on 'mouseover', (obj) ->
                   tooltip.transition().duration(20).style 'opacity', 1
-                  tooltip.html('<strong>x:</strong> ' + d.key[0] + '<br/><strong>y:</strong> ' + d.key[1]).style('left', d3.event.pageX + 5 + 'px').style 'top', d3.event.pageY - 35 + 'px'
-                .on 'mouseout', (d) ->
+                  tooltip.html('<strong>x:</strong> ' + obj.key[0] + '<br/><strong>y:</strong> ' + obj.key[1]).style('left', d3.event.pageX + 5 + 'px').style 'top', d3.event.pageY - 35 + 'px'
+                .on 'mouseout', (obj) ->
                   tooltip.transition().duration(50).style 'opacity', 0
-                .style 'opacity', (d) ->
-                  if d.key[0]? and d.key[1]? then 1 else 0
+                # Hide plot points where either the X or Y value is null. The
+                # data object's key 0 value is the X axis data value and the
+                # key 1 value is the Y axis data value.
+                .style 'opacity', (obj) ->
+                  if obj.key[0]? and obj.key[1]? then 1 else 0
 
           # Render the charts.
           dc.renderAll()

            .renderHorizontalGridLines true
            .symbolSize CHART_LAYOUT_PARAMS.symbolSize
            .symbol CHART_LAYOUT_PARAMS.subjectChartSymbol
            # TODO - What is category 10?
            .colors d3.scale.category10()
            # TODO - What is d here? Why key 1?
            .colorAccessor (d) -> d.key[1]
            .xAxisLabel "Patient"
            .yAxisLabel "Visit"
            .elasticX true

            yAxis = axes[i].y

            # Set up the dimension based on the X and Y axis user selections.
            dim = xFilter.dimension (obj) ->
              [
                obj[xAxis]
                obj[yAxis]
              ]

            # The chart group.
