quantmod: overlay plots

If you follow the first alternative suggested by Gei and use
PerformanceAnalytics, the RF function (which is an alias for
makeReturnFrame) in my qmao
package<https://r-forge.r-project.org/R/?group_id=1113> can
assist you in merging the data.

library(qmao)
library(PerformanceAnalytics)
charts.PerformanceSummary(RF(getSymbols(c("QQQ","EWG","EWI"))))

The chart_Series function (note the underscore) can also handle multiple
time series

library(quantmod)
getSymbols(c("SPY","DIA"))

# This will put 2 price series on a chart
thm <- chart_theme()
thm$col$line.col <- 'lightblue'
chart_Series(Cl(SPY),theme=thm)
add_Series(Cl(DIA),on=1)