Add support for mixed charts

[dvdkroft]

As of version 2.0 Chart.js supports mixed charts. For instance a bar chart mixed with a line chart. Would you add support for these kind of charts, please?

[JelteMX]

This requires some major refactoring, so don't expect it very soon. Maybe someone in the Community has time to build this feature request (currently I'm packed with other projects and issues)

[Mcpoowl]

@dvdkroft We just released the Google Combo Chart to the appstore. Different library, but might just do what you want!

[dvdkroft]

@Mcpoowl Thanks. Will check it out.

[dvdkroft]

I've sent in a pull request. Since this is my first please review it thoroughly. :-)

[JelteMX]

I have analysed your PR and declined it, because we're hacking the BarChart, but not implement it in for example the LineChart. See my comment. I thank you for your PR, will see if we can create a 'hybrid' version