I think this has something to do with the fact that the muted()
function that used to be a part of ggplot2 has migrated to the new
scales package. To help me clarify the issue, can you please let me
know what versions of ez and ggplot2 you are using? To get the version
number, just type ?ez or ?ggplot2 and look at the very bottom of the
help page that comes up.