Compute the coordinates to produce a tendril plot.
In the tendril plot, each tendril (branch) represents a type of events,
and the direction of the tendril is dictated by on which treatment arm the
event is occurring. If an event is occurring on the first of the two
specified treatment arms, the tendril bends in a clockwise direction.
If an event is occurring on the second of the treatment arms, the
tendril bends in an anti-clockwise direction.
Ref: Karpefors, M and Weatherall, J., "The Tendril Plot - a novel visual summary
of the incidence, significance and temporal aspects of adverse events in
clinical trials" - JAMIA 2018; 25(8): 1069-1073