The problem is a bit tricky, I will try to be as clear as possible. I have several lists (let's call them x, y, z and k), all of the same length, and where the i-th value of each list corresponds to a single observation (so for instance x[[3]], y[[3]], z[[3]] and k[[3]] are the values of four variables for the third observation). What I want to do is a scatter plot, where the x-axis reports the values of x and the y-axis the values of y. Now, for each pair (x[[i]], y[[i]]) I want a red dot if z[[i]] > k[[i]], and a black dot otherwise. Additional issue: for the first observations, the list k is empty, so I want to plot a red dot for such observations. Any idea on how is it possible to do it?