Only works for functions that return a single plot if by == NULL. This is not the case for some functions plotting results for all three categories, e.g. reliability diagrams or ROC curves.
create_diagram_by_level(FUN, by, dt, ...)
The name of the function creating the diagram
Column names in dt to group by
data table (cannot be part of ..., because a sub-data-table is passed to FUN)
arguments passed to FUN