Web support v3.0.0

[w-rui]

1. replace dependency 'webview_flutter' with 'webviewx'.
2. support web with webviewx.
3. fix white screen by ValueKey(_fixWhiteScreen).
4. update example android minSdkVersion to 19.

[dJani97]

@entronad

Hey!
Do you have plans on merging this? I'd really appreciate web support.

[entronad]

@dJani97

We are not going to replace webview_flutter with webviewx, for that webview_flutter is the official webview plugin.

If you need charting library in Flutter web, I suggest this one: Graphic

[SuricateTrading]

1. replace dependency 'webview_flutter' with 'webviewx'.

   2. support web with webviewx.

   3. fix white screen by ValueKey(_fixWhiteScreen).

   4. update example android minSdkVersion to 19.

Could you please send example code for a 3D chart?
I copied your commit and line charts work. But when I try to load a 3D chart nothing shows up.
Thank you very much!