plot3.R

##read data
ta<- read.table("household_power_consumption.txt", sep=";", header = TRUE, na.strings = "?")
newdata <- subset(ta, Date == "1/2/2007" | Date == "2/2/2007")

date<-as.character(newdata$Date)
time<-as.character(newdata$Time)

##paste the date and time into one string, then change it into POSIXlt.
datestr<-paste(date,time)
t<-strptime(datestr," %d/%m/%Y %H:%M:%S") 

##draw
with(newdata,plot(t,Sub_metering_1,type = "l", xlab="", ylab="Energy sub metering"))
with(newdata,lines(t,Sub_metering_2,type = "l", col = "red"))
with(newdata,lines(t,Sub_metering_3,type = "l", col = "blue"))

legend("topright",c("Sub_metering_1", "Sub_metering_2", "Sub_metering_3"), lty=1,col= c('black', 'red','blue')) 

##copy to file device
dev.copy(png, file = "plot3.png")
dev.off()