plotting visual stimulus on timeseries graph?

Hello!

I want to plot every stimulus onset as a multiple bars on the x axis on my timseries graph.

What kind of works so far is below:

onset = [100 120 130]

plot(time, timeseries); hold on
stem(ons, ones(1, length(ons)))

Which produces a graph like this:

but i want a graph that plots the onset times + 32 seconds as a line at the top of the graph:

any help would be much appreciated!