Chartjs-Angular 7, chart.js even when chart replaced, remaining chart in background which appears sometimes when mouseover

The behaviour I figured out is that the chart creation has to be done only one time.

And then the properties set like this.mainChart.data.datasets = [...] or this.mainChart.data.datasets.push (...)

Otherwise there seem to be a zombie chart remaining in background and appearing on mouseover.

Why?? big question….