Plots difference between two stocks

R Plot Differences

setwd('~/Desktop/R/Inputs')
getYahooCsv <- function(startDate, endDate, ticker){
  string <- paste("http://ichart.finance.yahoo.com/table.csv?s=",ticker,
                  "&a=",startDate[1]-1,"&b=",startDate[2],"&c=",startDate[3],
                  "&d=",endDate[1]-1,"&e=",endDate[2],"&f=",endDate[3],"&g=d&ignore=.csv",
                  sep="")
  
  return (read.csv(string))
}

getClose <- function(start, end, ticker){
	raw <- getYahooCsv(start, end, ticker)
	data <- raw[order(nrow(raw):1),]
	return( data$Close[1:nrow(data)] )
}

plotStockDif <- function(start, end, tick1, tick2){
	datapoints <- getClose(start, end, tick1) - getClose(start, end, tick2)
        plot(1:length(datapoints), datapoints, type='l')
}

plotStockDif(c(10,31,11),c(10,31,12),'F','GM')