Question

Give some data points plotted like this (below), how would I go about getting the intersecting values which intersect at a specific y. For example, if I were to draw the horizontal line y=1000, which x values on the plot would it intersect with (as an array)?

My data looks like this

x <- c(1, 2, 3, 4, ..., )
y <- c(1000, 1200, 900, 700, ..., )

Thank you.

Answer 1

Finding threshold crossings without smoothing rarely gives well-defined values. Play with the following:

data(sunspots)
sunspots = as.numeric(sunspots)
smoothover = 21 # Try smaller values here to see the failure
y = filter(sunspots,rep(1/smoothover,smoothover),circular=TRUE)
plot(y)
thresh =30
abline(h=thresh)
cross = which(diff(sign(y-thresh))!=0)-1
rug(cross)

If you want to get “exact” threshold crossings (what is exact if smoothing is so important?) you should use the above approximation first to get a region around the crossing and do a quadratic or linear interpolation afterwards, e.g. with 5 points.