I have 2 scripted fields: "ratio" (number) and "time_bucket" (string). These are showing up as expected in "discovery" without issue. I can also use the "ratio" field without issue when creating visualisations however I want to make some "sub-buckets" based upon the value of "time_bucket". But it doesn't appear in any list when trying to use it in "Visualisations", I cannot use it. Why is this?