score:2

after testing several times, this seems to fix the issue:

document.getelementbyid("btn1").addeventlistener("click",function(){
mypiechart.data.datasets[0].data = data1arr;
mypiechart.update();
});

document.getelementbyid("btn2").addeventlistener("click",function(){
mypiechart.data.datasets[0].data = data2arr;
mypiechart.update();
});

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