Help with custom fields in a plugin

[Nipunx1999]

So i was working on a plugin for boxplots, which support multiple values and split bys as well. While it is functional I am not able to wrap my head around how would i be able to pass some custom columns when i call viewer.save(), i have x-axis as a possible option in the config sidebar, but when i call viewer.save() i am not able to get this, i did try to see in the browser console via plugin.save() and it returns the extra fields which my plugin would use but does not return anything when i use viewer.save(). Any idea?