Question

If my X axis is time, and my Y is numeric data, how can I add a point at an arbitrary Y value (Say 500) whenever a point exists?

I am overlaying using lines on top of other plots.

Answer 1

Just add more points, with the same x values as your previous data, and a fixed y value.

So you have something like:

dfr <- data.frame(x = sample(100, 10, replace = TRUE), y = runif(10))
with(dfr, plot(x, y))

and you want to add

 points(dfr$x, rep.int(0.5, 10), col = "blue")

Having time for x values shouldn’t affect anything.