Hi, while researching to achieve the same thing, I stumbled upon this post and want to share my solution:
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Go into the Transform data tab, press “+ Add transformation” and choose “Add field from calculation”. Choose the Mode “binary operation”. Then create the operation “total data size” minus “used data size”. You can give your new field an alias name like “free storage”:
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Now you have three fields, but want to hide the field with the total size. This post helped me find a solution for that. Scroll the right panel down to the bottom and click “+ Add field override”. Select “Fields with name”. Choose your “total size”-field and press “+ Add override property”. Then select “Series > Hide in area”. To completely hide the data point, click on all three fields (tooltip, Viz and Legend). Now the “total size”-field should be hidden.
Hope this helps 