 					//plot the point to the current bitmapdata
 					renderer.plotPoint(point,dPt,ds.pollutant,selections.isSelected(point));
 					_prevPos[ds.dataURI] = dPt;
+					_prevVal[ds.dataURI] = point.value;
 				}
 				_prevMaxPtNums[ds.dataURI] = Math.max(i - 1,0);
 			}

		        else {
		            plotBitmapData.fillRect(new Rectangle(0,0,plotBitmapData.width,plotBitmapData.height),0x00000000);
		        }
		        _prevPos = {};	
			}
	        
			//prep the renderer we'll use to draw points
			var renderer:PointRenderer = new PointRenderer(plotBitmapData,pointDiameter,pointOverlapTolerance);
			
			plotBitmapData.lock();
			

	import etc.AirQualityConstants;
	import etc.DirtyFlag;
	import etc.PointRenderer;
	import etc.AirQualityColors;
	
	import events.DataPointEvent;
	
