Make another plot. (After all, they are dirty.) mtext , like most basic graphics functions, works with a one-sided build side effect model. There is no underlying repository that can be modified after the fact. (In the past I tried to write with the identical letters col="white" , but it looks awful.) If this is not the case, if you are on an OS that has the default "Save As .." that creates a pdf file, then you can use pdf editing software.