ui: prevent undefined axis label on custom chart

[zachlite]

Release note (bug fix): Y-axis labels on custom charts no longer display 'undefined'.

@thtruo, this fix addresses the issue as described in #72115.

Now, when the user selects a new unit from the dropdown, the user will experience up to a 10 second delay before the axis label refreshes. @nathanstilwell and I investigated this last week, and we'd need more time to think of a solution that doesn't create long term maintenance headaches. The benefit of this PR as it stands is that the Y-axis label is noticeably less broken. FYI.

[cockroach-teamcity]

This change is 

[nathanstilwell]

LGTM!

[zachlite]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from ac5dfab to blathers/backport-release-21.1-73055: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.1.x failed. See errors above.

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