you may see a full code example an app where a chart is updated with new data based on a selection in a Release dropdown here. In that code I check if chart exists, and remove it if it does before recreating it:

if (this.down('#myChart')) {
                        xtype: 'rallychart',
                        height: 400,
                        itemId: 'myChart',
                        chartConfig: {
                        chartData: {
                            categories: scheduleStateGroups, 
                            series: [ 
                this.down('#myChart')._unmask();  //otherwise loading mask does not go away

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