           chart.on 'renderlet', (chart) ->
             chart.selectAll '.symbol'
               .on 'mouseover', (d) ->
-                # TODO - What is div here?
-                # TODO - Remove the returns below if they are extraneous.
-                div.transition().duration(20).style('opacity', .9)
+                # The 'tooltip' 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] }"
                 #     ...
                 #     html = "#{ label }..."
-                #     div.html(html)
-                div.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'
-                return
+                #     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) ->
-                div.transition().duration(50).style('opacity', 0)
-                return
+                tooltip.transition().duration(50).style('opacity', 0)
 
           # The axes determine the number of charts to display.
           chartCnt = axes.length

          d3.selectAll('.chart-body').moveToFront()

          # The tooltip div.
          div = d3.select('body').append('div').attr('class', 'tooltip').style 'opacity', 0

      ]

            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

