xyplot() in R, spaghetti plot, how to make a tick at a certain point for each individual

Question

Good day,

I am using xyplot() to create a spaghetti plot... for each subject_identifier it plots the score of Y over time.

xyplot(Y~time,groups=subject_identifier,data=c,type=c('l'))

An event happens for each subject_identifier at some specified time, event_time. How can I make a tick or a dot on each line at event_time for each individual?

Thanks

Answer 1

If the event_times are a vector: event_times <- c(5,3,6,8) and you expect the groups to line up in order with those indices then you can use groups as an index into event_times which will then be indices for x and y in the panel arguments:

xyplot(Y~time,groups=subject_identifier,data=cdat,
              panel=function(x,y,groups,...){
                panel.xyplot(x,y,groups=groups,...)
                panel.segments(x0=x[(grp-1)*10+event_times[grp]],
                               x1=x[(grp-1)*10+event_times[grp]], 
                               y0=y[(grp-1)*10+event_times[grp]]-.2,
                             y1=y[(grp-1)*10+event_times[grp]]+.2, groups=groups,...)},
       type=c('l'))

So the first group gets ticked at time=5, the second at time=3 , and so on. Need to offset by (group-1)*10 although there may be cleaner approaches with panel.superpose. Other options for ticks are of course possible but you have been rather vague in your problem presentation.