feat(chart): support negative values in logarithmic axes

[andreasgerstmayr]

The logarithm of negative values is not defined.
However, in the case of a logarithmic chart axis, it makes sense to
define log(-x) as -log(x), to support logarithmic axes with negative
values.

Cf. 'symlog' in matplotlib

Resolves: #15558
Resolves: #17459

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Supports negative values in logarithmic axes.

Fixed issues

• Display negative value in Logarithmic axis #15558
• [Feature] Support negative and zero when axis type is 'log' #17459

Details

Before: What was the problem?

Axes with logarithmic scales didn't support negative values, because Math.log() of a negative number is undefined (NaN). Therefore the ticks on the yAxis disappeared entirely.

After: How is it fixed in this PR?

Negative values for logarithmic scales work by defining log(-x) = -log(x) and negative values are shown on logarithmic axes.

Misc

Related test cases or examples to use the new APIs

test/bar-log-negative.html

Others

Merging options

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

Other information

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[alexandrepola]

Hey @andreasgerstmayr, do you know if they have an estimated deadline for reviewing/merging the PR? I'm facing the same problem here

[andreasgerstmayr]

Hey @andreasgerstmayr, do you know if they have an estimated deadline for reviewing/merging the PR? I'm facing the same problem here

I don't know, I'd also like to get this PR merged :)

[octoth0rpe]

I would also love to see this merged. Is there anything we could do to help this along?

[Antti-Palola]

@octoth0rpe was there something still missing from this PR?

This feature would be highly appreciated ❤️

Tested this PR locally and it seems to be working well!

[octoth0rpe]

@octoth0rpe was there something still missing from this PR?

Nothing from my perspective, but I'm not an echarts maintainer 🤷 Just another user who wants this feature!

[Antti-Palola]

@Ovilia, it seems this PR has been lost in the pile of PRs. I checked that you had contributed to the log scaling before. Can you check this or maybe think of someone who could help make progress in this?

[Ovilia]

Thanks for the contribution!

There seems to be a bug with test/logScale.html, where the minimal value of the yAxis should not be a negative value since the data are all positive.

And please also add zero in the test case in bar-log-negative.

[Antti-Palola]

@andreasgerstmayr how are things on your plate? Are you able to fix that test page?

[andreasgerstmayr]

@andreasgerstmayr how are things on your plate? Are you able to fix that test page?

@Antti-Palola I gave it a shot the other day but either the new test/bar-log-negative.html test works, or test/logScale.html, but not both.
I'm busy with other projects at the moment, if you're interested in this feature and have time, feel free to look into it.