I believe there is a package called 'LSMEANS' that will compute the (wait for it...) LSMEANS. LSMEANS == adjusted means, it should be at the average value of the continuous covariate. One approach is to plot these LSMEANS. Another is to look at the differences betwixt them, and express everything as the difference from the control category, if there is a good choice of one. hack it a bit, the package is basically self explanatory as I recall but I have not used it in a while as I have been sequestered to SAS these days. GGPLOT2 is the happenin' plot package last time I checked...
Another good plot is just covariate on x by independant on y, color by treatment group.
Boxplots im not so sure about...