Chartjs-Passing text to a popup window opened in a JS function

You could pass basic data in the url hash, change your JS to

var popup=window.open('popup.html#MY_DATA', 'myWindow','width=500,height=500');

and then in the popup.html page you can access the it by looking into the location.hash variable:

window.location.hash
//This will be "#MY_DATA"