Combine two ggplots into one

gistfile1.r

# Original two
ggplot(US_Propane_Weekly, aes(as.Date(date), value)) + geom_line() + xlab("date") + ggtitle("US_Propane_Weekly")
ggplot(Iowa_Propane_Monthly, aes(as.Date(date), value)) + geom_line() + xlab("date") + ggtitle("Iowa_Propane_Monthly")

# Combine into one
p = ggplot(US_Propane_Weekly, aes(as.Date(date), value)) + geom_line() + xlab("date") + ggtitle("US_Propane_Weekly vs. Iowa_Propane_Monthly")
p = p + geom_line(data = Iowa, aes(as.Date(date), value), colour="red")
p