Hi Adam,
I’m Jung and I’d be happy to help you out with your question.
Just to set your expectations that we are sorry to say that in Excel for web that currently, it is not available yet. It can only achieve in Excel desktop app version.
In Excel online as a workaround to change axis label in chart here are steps.
1.Click the cell that has the label text you want to change.
2.Type the text you want, and press Enter.
The axis labels in the chart are automatically updated with the new text.
For you reference: https://support.microsoft.com/en-us/office/change-axis-labels-in-a-chart-2355f3ef-d609-4e21-9537-552f0c81d435?ui=en-us&rs=en-us&ad=us
https://support.microsoft.com/en-us/topic/change-the-scale-of-the-horizontal-category-axis-in-a-chart-637897f6-0d51-4ec5-bef9-25d2c83a8450
https://support.microsoft.com/en-us/office/change-axis-labels-in-a-chart-in-office-1efa2bd8-2b46-4db1-b15e-c8fd12e39a8c.
We highly appreciate you understanding
I hope this information helps. If you have any questions, please let me know and I’ll be glad to assist you further.
Best regards,
Jung