Allow conditional colouring for lines in bidimensional line graphs

[KaelanFrey]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

The end user should be able to see a single line graph with different colours depending on a line segment's starting Y value.

What does the proposed API look like?

Usage example, if we wanted to colour depending on a Y value called "value", pivoted by a "date":

const lineView = chart.createView({
                padding: 32
            });
            lineView.data(data.lineData);
            lineView.scale('value', {
                alias: 'Value'
            });
            lineView.line()
                .position('date*value')
                .color('_*value', (_,v) => v > 40000 ? 'green' : 'red');

That would create a view where each line that would start on a point with a value higher than 40000 would be green, or red if not.
This is exactly how points handle colouring, but the same logic is not applicable to lines.

EDIT: Currently, calling .color like that will colour the whole line depending on the value of the last value. It should be possible to colour segments of the line. A final example would be something like this:

[hustcc]

Drawing a line with a single path, so can not set the color for different path.

You can use chart.annotation to finish your work. see the demo https://g2.antv.vision/zh/examples/component/annotation#guide-region-filter1