overlaying mutations on an antimicrobial susceptibility (MIC) barplot

Hello, I am presenting some microbial genomics data in ggplot2. Has anyone tried to superimpose mutations on an MIC distribution barplot? I'm anticipating a barplot of the frequency of each doubling dilutions, and then superimposed on the bars, all the mutations observed in a particular resistance-associated gene at each doubling dilution. There are often multiple overlapping mutations so I would only be looking to represent a 'list' of mutations that occur, rather than a stacked barplot representing relative frequency