           # Pad the left margins.
           chart.margins().left += CHART_LAYOUT_PARAMS.leftMargin
           
-          # Supply the tooltips.
+          # Apply a renderlet and associate it with tooltip callbacks.
           chart.on 'renderlet', (chart) ->
             chart.selectAll '.symbol'
-              .on 'mouseover', (d) ->
-                # The 'tooltip' is the selection returned by d3 after the chart
-                # rendering call.
+              .on 'mouseover', (obj) ->
+                # Supply the tooltips. The tooltip element is the selection
+                # returned by d3 after the chart rendering call.
                 tooltip.transition().duration(20).style('opacity', .9)
                 # TODO - make temp variables and interpolate the argument,
                 #   e.g.:
-                #     label = "<strong>Patient:</strong>#{ d.key[0] }"
+                #     label = "<strong>Patient:</strong>#{ obj.key[0] }"
                 #     ...
                 #     html = "#{ label }..."
                 #     tooltip.html(html)
-                tooltip.html('<strong>Patient:</strong> ' + d.key[0] + '<br/><strong>Visit:</strong> ' + d.key[1]).style('left', d3.event.pageX + 5 + 'px').style 'top', d3.event.pageY - 35 + 'px'
-              .on 'mouseout', (d) ->
+                tooltip.html('<strong>Patient:</strong> ' + obj.key[0] + '<br/><strong>Visit:</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)
 
           # The axes determine the number of charts to display.

                .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) ->
                  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.transition().duration(50).style 'opacity', 0
                .style 'opacity', (d) ->
                  if d.key[0]? and d.key[1]? then 1 else 0

          # Render the charts.
          dc.renderAll()

            .renderHorizontalGridLines true
            .symbolSize CHART_LAYOUT_PARAMS.symbolSize
            .symbol CHART_LAYOUT_PARAMS.subjectChartSymbol
            # Construct a new scale with a range of ten categorical colors.
            .colors d3.scale.category10()
            # The symbol color is based on the data object's key 1 value, which
            # is the session number.
            .colorAccessor (obj) -> obj.key[1]
            .xAxisLabel "Patient"
            .yAxisLabel "Visit"
            .elasticX true
