feat(line): add monotone cubic Hermite smoothing

[awtaylor]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others
• [ ]

What does this PR do?

Add an opt-in monotone cubic Hermite smoother for cartesian line and area series.

Fixed issues

• [Feature] Monotone cubic Hermite smoothing for cartesian line series #21793
• [Bug] smoothMonotone generates saddle points #21532

Details

Before: What was the problem?

smooth: true can bow past the samples. smoothMonotone: 'x' and 'y' avoid that by flattening the tangent at every sample, which draws a saddle or plateau at each knot (#21532, #7158).

After: How does it behave after the fixing?

smoothMonotone: 'monotoneX' and 'monotoneY' use Fritsch–Butland tangents, so the curve passes through the samples without adding extrema between them. smooth: true is full strength in this mode; a number in (0, 1] scales the tangents. The default and the existing 'x', 'y', and 'none' modes are unchanged. On polar series the new values fall back to the current Bézier smooth.

Document Info

• 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

Security Checking

• This PR uses security-sensitive Web APIs.

ZRender Changes

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

Related test cases or examples to use the new APIs

• test/ut/spec/chart/line/monotoneCubic.test.ts
• test/line-monotone.html

Merging options

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

Other information

Interior slopes are Fritsch–Butland (1984), inside the Fritsch–Carlson monotone region (1980). Steffen (1990) is the d3/Vega alternative and can replace the tangent function without an API change if that look is preferred.

[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.

Please DO NOT commit the files in dist, i18n, and ssr/client/dist folders in a non-release pull request. These folders are for release use only.

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.