Plot a function in R

I have two continuous variables x and y, and I need to plot the points in different colors according to a third factor variable called DIAGNOSI (that is, according to the disease group these observations belong) and used this: m<-qplot( x = Qalb, y = Kratio, data = dataset, color = dataset$DIAGNOSI ) Then, there is an index whose optimal cutoff has been identified in 2.43, and this index is equal to y/x, so i need to add to the plot a line showing where this threshold is i need the remaining code for this