Fix prepareCustoms key for Single axis.

[benlongo]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

When using a custom series with a single axis, a lookup on prepareCustoms fails as it is looking for single instead of singleAxis.

Fixed issues

• custom series on single axis now works

Related test cases or examples to use the new APIs

If this is the right approach to fixing this, I can put together a reproduction and test for it.

[echarts-bot]

Thanks for your contribution!
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[pissang]

@benlongo Thanks for the contribution and LGTM!. Adding a new test case for this fix will be great! You can append this test case to test/custom.html

[benlongo]

Sorry for the long delay! I've added a test and confirmed that it is broken on master and that this branch fixes it. I additionally fixed a typo I came across but can pull that into a separate PR if you prefer.

I modified an existing example to reproduce the bug, which isn't a minimal, but I can make it simpler if required.

[pissang]

@benlongo It's fine. All looks good! Well done!

[echarts-bot]

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