fix(axisPointer): always show for connected charts with null values

[sjcobb]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Add axisPointer.alwaysShowContent: true option to force axisPointer to show when charts are connected. A common case is for all time series charts on a dashboard to share an xAxis axisPointer to correlate across charts when hovering. Currently this does not work if series data is aligned differently / has null values, the axisPointer flickers instead of showing at the correct timestamp

Fixed issues

• [Bug] Axis Pointer for connected charts with null values dosen't sync properly between charts #20058
• [Bug] AxisPointer sync not work acros multiple synced graphs #20823

Details

Before: What was the problem?

Inconsistent display of axisPointer for charts using 'connect':

before_connect_always_fix.mov

After: How does it behave after the fixing?

axisPointer always shows on all connected charts:

after_connect_always_fix.mov

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]

Consider adding axisPointer.alwaysShowContent as tooltip.alwaysShowContent.

[sjcobb]

Consider adding axisPointer.alwaysShowContent as tooltip.alwaysShowContent.

Thanks for the feedback! This is fixed now to be axisPointer.alwaysShowContent

[DavidNepozitek]

Hi @Ovilia,
can we get this reviewed?
It is a small change that really improves experience when designing dashboards as stated in the original post.

Thanks!