feat(axisPointer): keep pointer on connected charts without data

[aggarwal-h]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Adds axisPointer.triggerOnNoData so connected charts keep showing the axisPointer at the linked axis value even where they have no data, instead of hiding it.

Details

Before: What was the problem?

With echarts.connect, hovering over one chart replays the action on the others. On a connected chart, the original pixel point is meaningless, so the point is treated as illegal. When that chart also has no data at the sample location, the point stays illegal, shouldHide becomes true, and the axisPointer is hidden, even though the hovered axis value is known and the chart has a valid axis range. The pointer disappears on the chart with no data, which is confusing for synced time-series dashboards.

After: How does it behave after the fixing?

Two opt-in options on axisPointer (both default to current behavior):

• triggerOnNoData (default false): when true, the axis is still processed for an illegal point (e.g. on a connected chart) as long as a value is available from the input axesInfo (the linked value) and it is not a leave event. The pointer is positioned by the axis value, independent of series data. The leave guard and the input-axis restriction are preserved, so hide-on-leave and dispatchAction axis targeting are unchanged.
• findPointOnConnectedCharts (default true): chart-wide option to skip looking up a sample series point when the incoming point is illegal, so the pointer follows the linked value instead of snapping to a sample data point.

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

(New options axisPointer.triggerOnNoData and axisPointer.findPointOnConnectedCharts need entries in apache/echarts-doc.)

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/axisPointer-triggerOnNoData.html

Merging options

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

[echarts-bot]

Thanks for your contribution!
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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.