you can simply use $.get() or $.getjson() method and in url add parameter, for example:

$.getjson('' + window.index, function(data) {        
    //create new chart with data from response

then in your getpie.php you have access to this variable via:

$clickedindex = $_get['index'];

the same way works it for highcharts example lazy-loading and php file you can find here.

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