So, I managed to create this beauty...
(Sorry I cut off the legend - the individual lines are for different years - explained below).
I have documents with field user_id and then I have scripted fields year and month_of_year which are calculated off of @timestamp.
I defined a line graph where I first did a split lines on term year and then a sub-bucket x-axis on term month_of_year. In both cases I ordered by Term Ascending (with 6 for the year, since my data is 2011-2016, and 12 for the month_of_year). The y-axis is unique count of user_id.
As you can see, something is goofy. Specifically, it seems to have had trouble sorting month_of_year in ascending order. Is this a bug, or did I just configure something incorrectly?
FYI, my goal for this graph is to have a line for each year showing number of unique users by month - for easy year-over-year comparison.