[new chart type]: combined chart #22

@issuehunt has funded $50.00 to this issue.

• Submit pull request via IssueHunt to receive this reward.
• Want to contribute? Chip in to this issue via IssueHunt.
• Checkout the IssueHunt Issue Explorer to see more funded issues.
• Need help from developers? Add your repository on IssueHunt to raise funds.

So, I'm planning to make a new Combined.js in the src and then build it as a separate new chart or you want me to re-use the Bar and Line code, is that possible in some way?

@gurrrung Thanks for the interest, I think we should re-use the Bar and Line code in some way, so we won't need to copy-paste code around when the charts changes(e.g. https://github.com/timqian/chart.xkcd/issues/27), but the current implementation of them is not easy to be reused, we may need to refactor the code a little

Suggestions on the refactoring are welcomed

Ref: How chart.js implement mixed type chart https://www.chartjs.org/docs/latest/charts/mixed.html