feat(line): line segment styling

[pe-3]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

This PR will add line segment styling to line chart.

Fixed issues

#17138

Details

Before: What was the problem?

We cannot set segment styles for a line chart, so the entire line has to use the same style, making it monotonous. To achieve this effect, we need to increase the complexity of the configuration, which is not elegant. Therefore, at the source code level, we need to implement this effect to provide users with a direct configuration option to easily configure such segmented effects.

After: How does it behave after the fixing?

Users can configure segment styles like this

{
    linePieces: [
         { 
              segment: [1, 3],
              lineStyle: {
                   type: "solid",
                   color: 'red'
              }
         },
        {
             segment: ({dataIndex})= dataIndex >= 6 && dataIndex <= 9,
             lineStyle: {
                   type: "dashed",
                   color: 'cyan',
                   width:'4'
             }
        }
    ]
}

The effect will be like this：

Document Info

One of the following should be checked.

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

ZRender Changes

• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

N.A.

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

[echarts-bot]

Thanks for your contribution!
The community will review it ASAP. In the meanwhile, please checkout the coding standard and Wiki about How to make a pull request.

Document changes are required in this PR. Please also make a PR to apache/echarts-doc for document changes and update the issue id in the PR description. When the doc PR is merged, the maintainers will remove the PR: awaiting doc label.

[Ovilia]

What does segment: [1, 3] mean? The 1st and the 3rd line or the lines between the 1st and the 3rd data?

[pe-3]

Represent the line segment to be selected with the index of the start point and end point.

[Ovilia]

This PR seems not to be ready yet. It can be marked as a draft.

[yassilah]

This feature would be extremely useful at least to be able to style the line to connect null values. I'm happy to take it over if need be