A number of organizations use Highcharts to make charts. Highcharts requires a licensing fee when it’s used outside personal or nonprofit projects. D3 and C3 are open-source and free. So if you need to create flexible charts for a for-profit company, and they won’t pay for a Highcharts license, welcome to C3.
I like introducing students to charts with Highcharts because of the excellent JSfiddle examples provided. We don’t have that with C3 — or D3. D3 has a very steep learning curve. The best resource I have found for learning D3 is this series of 20 videos (free).