Better documentation for custom charts

[martin-smith]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[X ] Documentation issue or request
[ ] Other... Please describe:

Current behavior

Currently in the docs there is little in the way of semi advanced custom charts.
The documentation only has simple charts and then jumps immediately to the complex radar example

Expected behavior

What would be ideal is a series of documentation detailing almost "my first" custom charts. Likely covering the below topics

1. X/Y scaling -> or linking to other references (as the current docs doesn't really explain)
2. Simple example for instance a simple BarSeries joined with a simple LineSeries - eg data with trendline
3. How axis work in relation to custom charts

What is the motivation / use case for changing the behavior?

This small amount of documentation would help bridge the gap for novice users, as whilst the library is very powerful it has a steep learning curve outside of simple usage. Furthermore as this library is not super well used there is practically zero support on places such as reddit or stackoverflow.