plot4.R

# Read in and name the data defining the period subset of interest

power <- read.table("household_power_consumption.txt", , na.strings="?")
names(power) <- c("Date","Time","Global_active_power","Global_reactive_power","Voltage","Global_intensity","Sub_metering_1","Sub_metering_2","Sub_metering_3")
subpower <- subset(power,power$Date=="1/2/2007" | power$Date =="2/2/2007")

# Transform the Date and Time vars from characters into objects of type Date and POSIXlt respectively

subpower$Date <- as.Date(subpower$Date, format="%d/%m/%Y")
subpower$Time <- strptime(subpower$Time, format="%H:%M:%S")
subpower[1:1440,"Time"] <- format(subpower[1:1440,"Time"],"2007-02-01 %H:%M:%S")
subpower[1441:2880,"Time"] <- format(subpower[1441:2880,"Time"],"2007-02-02 %H:%M:%S")


# initiate a composite plot with many graphs

par(mfrow=c(2,2))

# create a graph with 4 different sub graphs plotted

(subpower,{
  plot(subpower$Time,as.numeric(as.character(subpower$Global_active_power)),type="l",  xlab="",ylab="Global Active Power")  
  plot(subpower$Time,as.numeric(as.character(subpower$Voltage)), type="l",xlab="datetime",ylab="Voltage")
  plot(subpower$Time,subpower$Sub_metering_1,type="n",xlab="",ylab="Energy sub metering")
   with(subpower,lines(Time,as.numeric(as.character(Sub_metering_1))))
   with(subpower,lines(Time,as.numeric(as.character(Sub_metering_2)),col="red"))
   with(subpower,lines(Time,as.numeric(as.character(Sub_metering_3)),col="blue"))
   legend("topright", lty=1, col=c("black","red","blue"),legend=c("Sub_metering_1","Sub_metering_2","Sub_metering_3"), cex = 0.6)
  plot(subpower$Time,as.numeric(as.character(subpower$Global_reactive_power)),type="l",xlab="datetime",ylab="Global_reactive_power")
})

# create a .png version of the graph
dev.copy(png, filename="plot4.png")
dev.off()