HighCharts is not open source (chartjs#4383)

According to the (issue tracker of highcharts)[highcharts/highcharts#4405 (comment)] they do not consider the code open source. It's available for reading but that's about it.

The code is provided under CC-BY-NC via github.

See also:
https://shop.highsoft.com/faq#Non-Commercial-0

docs/notes/comparison.md

@@ -12,7 +12,7 @@ Library Features
 | Extendable to Custom Charts | ✓ | ✓ | |  |
 | Supports Modern Browsers | ✓ | ✓ | ✓ | ✓ |
 | Extensive Documentation | ✓ | ✓ | ✓ | ✓ |
-| Open Source | ✓ | ✓ | ✓ | ✓ |
+| Open Source | ✓ | ✓ | | ✓ |
 
 Built in Chart Types