Adding variable reference lines to twoway graphs

[Robert Bonow]

Hello,

I am trying to add vertical reference bars to a series of graphs plotted using xtline. I have a time-dependent variable, time_var, plotted versus day for individuals identified by id. I have been able to plot these in separate panels for each id using the following code:

xtline time_var, t(day) i(id)

I would like to add a vertical reference bar to each panel that is different for each id so that I can mark the time when an event occurs, event_day. For example, based on the data below, there should be a vertical line at x = 2 for id#2 and at x = 1 for id#3, but no vertical line for id#1.

I'm able to generate fixed reference lines that appear in all panels by using xline(), but I haven't been able to figure out how to set the placement of these lines equal to event_day. (xline(event_day) does not work...)

Should I reshape the data back to wide? Or is there a better command for what I'm trying to do?

Thank you!

id	time_var	day	event_day
1	5	1	.
1	6	2	.
2	34	1	2
2	50	2	2
2	60	3	2
3	2	1	1
3	2	2	1

[Andrew Musau]

To the extent that such a reference bar is only for visual purposes i.e. just serves to show when an event occurs, I would use the addplot option with the plottype dropline. To ensure that the line covers the entire length of the graph, create a variable that is equal to the maximum value of your variable of interest (in this case max time_var=60)

input id time_var day event_day
1 5 1 .
1 6 2 . 
2 34 1 2
2 50 2 2
2 60 3 2
3 2 1 1
3 2 2 1
end

xtset id day
gen max=60
 xtline time_var, addplot( dropline max event_day)