Like this?
set.seed(1)
df.1 <- data.frame(currency=rep("CHF/JPY",31),
price=rnorm(31,60,2),
volume=rnorm(31,5e5,1e5),
datetime=as.POSIXct("2011-09-06 08:00:00")+seq(0,1800,by=60))
df.2 <- data.frame(currency=rep("AUD-USD",121),
price=rnorm(121,50,3),
volume=rnorm(121,1e6,1e5),
datetime=as.POSIXct("2011-09-06 07:00:00")+seq(0,7200,by=60))
mydata <- rbind(df.1,df.2)
library(reshape2)
library(ggplot2)
gg <- melt(mydata, id=c(1,4))
ggplot(gg) +
geom_line(aes(x=datetime, y=value, color=currency)) +
stat_smooth(aes(x=datetime, y=value, color=currency),
formula=y~1,method="lm", se=F, linetype=2)+
facet_grid(variable~., scales="free")
Edit In response to OP's comment.
ggplot(mydata, aes(x=datetime, y=price, color=currency)) +
geom_line()+
stat_smooth(formula=y~1,method="lm", se=F, linetype=2)+
facet_grid(currency~., scales="free")