plot4.R

plot4 <- function(){
  
  # read the data file, then get the requried days
  data <- read.table("household_power_consumption.txt", sep = ";", header = TRUE, na.strings = "?")
  days <- data[which(data$Date == "1/2/2007" | data$Date == "2/2/2007"),]
  
  days[,"DateTime"] <- as.POSIXct(paste(days$Date, days$Time), format="%d/%m/%Y %H:%M:%S")
  
  # set up 2 x 2 grid in the device
  png("plot4.png")
  par(mfrow=c(2,2))
  
  
  with(days, plot(DateTime, Global_active_power, type="l", ylab="Global Active Power", xlab=""))
  with(days, plot(DateTime, Voltage, type="l", ylab="Voltage", xlab="datetime"))
  
  with(days, plot(DateTime, Sub_metering_1, type="n", ylab="Energy sub metering", xlab=""))  
  points(days$DateTime, days$Sub_metering_1, type="l")
  points(days$DateTime, days$Sub_metering_2, type="l", col="red")
  points(days$DateTime, days$Sub_metering_3, type="l", col="blue")
  legend(legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"), col = c("black", "red", "blue"),lty=1, x = "topright")

  with(days, plot(DateTime, Global_reactive_power, type="l", xlab="datetime"))
  
  dev.off()
}