add mouse-focus scaling signal

[fawdlstty]

This PR adds two default signals for scrolling the mouse wheel when the mouse pointer is over the chart, which will focus on the mouse position and then zoom freely, which is more in line with user habits than the original signal.
Run test:

import 'package:flutter/material.dart';
import 'package:graphic/graphic.dart';

import '../data.dart';

class LineAreaPointPage extends StatelessWidget {
  const LineAreaPointPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Line and Area Element'),
      ),
      backgroundColor: Colors.white,
      body: Column(children: [
        Center(
          child: Container(
            margin: const EdgeInsets.only(top: 10),
            width: 800,
            height: 800,
            child: Chart(
              data: scatterData,
              variables: {
                '0': Variable(
                  accessor: (List datum) => datum[0] as num,
                  scale: LinearScale(min: -200, max: 200),
                ),
                '1': Variable(
                  accessor: (List datum) => datum[1] as num,
                  scale: LinearScale(min: -200, max: 200),
                ),
                '2': Variable(accessor: (List datum) => datum[2] as num),
                '4': Variable(accessor: (List datum) => datum[4].toString()),
              },
              elements: [
                PointElement(
                  size: SizeAttr(variable: '2', values: [10, 5]),
                  color: ColorAttr(
                    variable: '4',
                    values: Defaults.colors10,
                    updaters: {
                      'choose': {true: (_) => Colors.red}
                    },
                  ),
                  shape: ShapeAttr(variable: '4', values: [
                    CircleShape(hollow: false),
                    CircleShape(hollow: false),
                  ]),
                )
              ],
              axes: [
                Defaults.horizontalAxis
                  ..position = 0
                  ..grid = Defaults.strokeStyle
                  ..line = Defaults.strokeStyle,
                Defaults.verticalAxis
                  ..position = 0
                  ..grid = Defaults.strokeStyle
                  ..line = Defaults.strokeStyle,
              ],
              coord: RectCoord(
                horizontalRange: [0, 1],
                verticalRange: [0, 1],
                horizontalRangeUpdater: Defaults.horizontalRangeFocusSignal,
                verticalRangeUpdater: Defaults.verticalRangeFocusSignal,
              ),
              selections: {'choose': PointSelection(toggle: true)},
              tooltip: TooltipGuide(
                anchor: (_) => Offset.zero,
                align: Alignment.bottomRight,
                multiTuples: true,
              ),
            ),
          ),
        ),
      ]),
    );
  }
}

[entronad]

Thanks!

[entronad]

Published in v1.0.1