CombinedChartRenderer property : _renderers should be set open

[opann]

• I've read, understood, and done my best to follow the *CONTRIBUTING guidelines.

What did you do?

ℹ Try to replace CombinedChartRenderer using custom LineChartRenderer

What did you expect to happen?

ℹ I override the function createRenderers() and replace the LineChartRenderer
, but I found out that it still using the default renderers to draw charts because _renderers
cant be override. So I change the _renderers property prefix from "internal" to "open"
Issue Fixed.
I recommend to change the _renderers property prefix to "open" that makes people can
using custom CombinedChartRenderer

What happened instead?

ℹ _renderers still generate default renderers so I cant using my own LineChartRenderer

Charts Environment

**Charts version/Branch/Commit Number:3.3.0
**Xcode version: 10.3
**Swift version:4.2
**Platform(s) running Charts:iPhone 8
**macOS version running Xcode:10.14.6

Demo Project

ℹ Please link to or upload a project we can download that reproduces the issue.

[liuxuan30]

if you are heavily customizing the library, you should grab the source code and make any modifications you want.

the renderers are not intended for any user to override.

and, I think CombinedChartRenderer and CombinedChartView are the classes you should override. You can easily replace the renderer within initialize() in CombinedChartView, and then use your own line chart renderer.