feat: allow areaStyle.origin to take number as input

[jiawulin001]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Allow series-line.areaStyle.origin to take number as input

Fixed issues

• Close [Feature] Area Chart with limit line where area fill is on both sides of mark line. #16717

Details

Before: What was the problem?

The choices for series-line.areaStyle.origin for now is limited to 'start', 'end', and 'auto', which correspond to baseline at bottom, top or axis of the view.

• origin: 'start'

• origin: 'end'

• origin: 'auto'

• And now what is required is to let origin accept a number and set the baseline right at the specified level. Take the picture below as an exmaple.
  

After: How is it fixed in this PR?

• What is changed in code?
• Here a second else if is added to take number in and set it to valueStart. NaN is excluded from such situation and will be treated as undefined.

function getValueStart(valueAxis, valueOrigin) {
      var valueStart = 0;
      var extent = valueAxis.scale.getExtent();

      if (valueOrigin === 'start') {
        valueStart = extent[0];
      } else if (valueOrigin === 'end') {
        valueStart = extent[1];
      } // If origin is specified as a number, use it as
      // valueStart directly
      else if (isNumber(valueOrigin) && !isNaN(valueOrigin)) {
          valueStart = valueOrigin;
        } // auto
        else {
            // Both positive
            if (extent[0] > 0) {
              valueStart = extent[0];
            } // Both negative
            else if (extent[1] < 0) {
                valueStart = extent[1];
              } // If is one positive, and one negative, onZero shall be true

          }

      return valueStart;
    }

• An example of using number as areaStyle.origin

Misc

• The API has been changed (apache/echarts-doc#xxx).

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

• The echarts-doc has not been changed yet, I can submit another PR to update the doc.

[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.

[pissang]

@jiawulin001 Wow looks great!

BTW do you mind adding a test case?

[jiawulin001]

yes, I can add a test case

[jiawulin001]

@pissang Any suggestion on which test.ts to add the test case to? I didn't find any existent test file that includes areaStyle, so my plan is to add a new test file under test/ut/spec/series. Is that OK?

[pissang]

@jiawulin001 We usually add test cases in the test folder. Run npm run mktest area-origin and it will generate a template for you.

[jiawulin001]

@pissang Hi, the test case has been added. Check it out!

[echarts-bot]

Congratulations! Your PR has been merged. Thanks for your contribution! 👍

[plainheart]

Hi @jiawulin001, thanks for your contribution to this new feature and this is just a reminder. Are you working on adding or planning to add this new area origin value for the line series in the documentation?

[jiawulin001]

@plainheart Oh yes, thank you for your reminding! The PR for updating documentation has been submitted.