I am creating a line chart in Kibana. My index pattern has a numeric value, @timestamp, and numerous keywords. I am trying to sort by time, put the sum of values on the Y axis, and split the time series into separate lines. Specifically, I'm trying to split by 4 of the keywords. I am able to create 3 "Term"-based series splits without issue, but when I click on "Add sub-buckets" again to create the 4th, it only lets me split the chart, not the series. I've tried all numerous orderings/combos of the 3 out of my 4 desired keyword splits that it will let me split by, but regardless of what I do, I can't create the 4th split.
3 seems like a pretty small and arbitrary number to limit users by for splits, and after a few hours of searching, I don't see that documented anywhere. Am I missing something here or is this just a known, but undocumented limit.
Kibana & ES 7.0.1.