I played around with this some more. The fix was pretty easy.. just
add one line resultchart.hideData(). At least this worked for me.
public function appendInterval(column:String):void
{
resultchart.hideData();
var series:ColumnSeries = new ColumnSeries() ;
series.xField = 'label' ;
series.yField = column ;
series.setStyle("showDataEffect", slideIn);
series.setStyle("hideDataEffect", slideOut);
var currentSeries:Array = resultchart.series;
currentSeries.push(series) ;
resultchart.series = currentSeries ;
}