As indicated at https://shop.highsoft.com/highcharts/,
"If you are a non-profit company, or use our products for personal use, 
you may enjoy our software for free under a 
Creative Commons (CC) Attribution-NonCommercial licence",
which is linked to:
https://creativecommons.org/licenses/by-nc/3.0/

Also note the license statement we received from HighSoft AB