[Road Map] Custom renderer for graphic walker

[ObservedObserver]

pygwalker-ob.mp4

graphic walker should be able to use any chart library as renderer, that's a feature designed since day1 of graphic-walker.
If you take a look of some of the data structured of graphic-walker, the early viz spec is more general instead of vega-lite spec. The prototype of graphic-walker is actually built with antvis/g2, since it is the only viz lib at that time supporting coordinate in grammar of graphics, allowing cool features like pie chart = polar_coordinate(stack bar chart) possible.

But I do decide to go with more vega-lite way since I really like the signals de# vega(like rxjs) and its ability to write decleartive interactions.

When graphic-walker/pygwalker get more users, people are talking about more customization on graphics, which vega-lite can hardly support. I still prefer vega-lite as default renderer for its extension of traditional grammar of graphics. That's when I decide the customizable renderer is necessary. Thanks for graphic-walker abstaction, which make the replacement of the renderer is not difficult.

But there are still a lot of shit code merged need to handle, still working on it.