plot1.R

library(data.table)
all <- fread('household_power_consumption.txt', na.strings=c("?"), colClasses=c('character', 'character', rep('numeric',7)))
## create a subset for the given 2 days
days <- subset(all, all$Date == '1/2/2007' | all$Date == '2/2/2007')

## for some reason the numerics were still converted to characters even with na.strings attributes
## so cast the values to numerics
days$Global_active_power <- as.numeric(days$Global_active_power)

## print it to file.
png("plot1.png", width = 480, height = 480)
hist(days$Global_active_power, xlab = 'Gobal Active Power (kilowatts)', col="red", main='Global Active Power')
dev.off()