Summary

 

This chart was created with R.

Figure is produced using the R statistical package, using the following code. It first reads the HTML directly from the website, then parses the data and saves the graph into your working directory. It should be able to be run directly by anyone with R.

#Load the complete HTML file into memory
html <- readLines(url("http://en.wikipedia.org/wiki/Opinion_polling_for_the_New_Zealand_general_election,_2008"),encoding="UTF-8")
closeAllConnections()
#The fourth table is the opinion poll data
tbl <- html[(grep("<table.*",html)[4]):(grep("</table.*",html)[4])]
#Now split it into the rows, based on the <tr> tag
tbl.rows <- split(tbl,cumsum(tbl=="<tr>"))
#Now extract the data
survey.dat <- lapply(tbl.rows,function(x) {
  #Start by only considering where we have <td> tags
  td.tags <- x[grep("<td",x)]
  #Polling data appears in columns 3-6
  dat     <- td.tags[3:6]
  #Now strip the data and covert to numeric format
  dat     <- gsub("<td>|</td>","",dat)
  dat     <- gsub("%","",dat)
  dat     <- gsub("-","0",dat)
  dat     <- as.numeric(dat)
  #Getting the date strings is a little harder. The approach we will take is to take advantage
  #of the title="date" hyperlinks to generate a set of dates
  date.str <- td.tags[2]                        #Dates are in the second column
  date.str <- gsub("<sup.*</sup>","",date.str)   #Throw out anything between superscript tags, as its an reference to the source
  titles <- gregexpr("(?U)title=\".*\"",date.str,perl=TRUE)[[1]]    #Find the location of the title tags
  #Now, extract the actual date strings
  date.strings <- rep(NULL,length(titles))
  for(i in 1:length(titles)) {
        date.strings[i] <- substr(date.str,titles[i]+7,titles[i]+attr(titles,"match.length")[i]-2)
  }
  yr <- rev(date.strings)[1]  
  dates <- rep(as.POSIXct(Sys.time()),length(date.strings)-1)
  for(i in 1:(length(date.strings)-1)) {
        dates[i] <- as.POSIXct(strptime(paste(date.strings[i],yr),"%B %d %Y"))
  }
  survey.time <- mean(dates)
  #And now return results
#  if(is.na(survey.time)) browser()
  return(data.frame(Date=survey.time,t(dat)))
})
polls <- do.call(rbind,survey.dat)

#Now make the graph!
polls <- subset(polls,!is.na(polls$Date))
polls$Date  <- as.double(polls$Date)
colnames(polls) <- c("Date","Helen Clark","Don Brash","John Key","Winston Peters")
parties <- colnames(polls)[-1]
cols    <- c("red","purple","blue","black")
ticks <- ISOdate(c(2005,rep(2006,3),rep(2007,3),rep(2008,3)),c(9,rep(c(1,5,9),3)),1)
xlims <- range(as.double(c(ticks,ISOdate(2009,2,1))))
png("NZ_opinion_polls_2005-2008 -PPM.png",width=778,height=487,pointsize=16)
par(mar=c(3,4,1,1))
matplot(polls$Date,polls[,parties],pch=NA,xlim=xlims,ylab="Party support (%)",xlab="",col=cols,xaxt="n",ylim=c(0,60))
abline(h=seq(0,95,by=5),col="lightgrey",lty=3)
abline(v=as.double(ticks),col="lightgrey",lty=3)
#Now add loess smoothers
smoothed <- list()
for(i in 1:length(parties)) {
  smoother <- loess(polls[,i+1] ~ polls[,"Date"],span=0.33)
  smoothed[[i]] <- predict(smoother,se=TRUE)
#  polygon(c(polls[,"Date"],rev(polls[,"Date"])),
#    c(smoothed[[i]]$fit+smoothed[[i]]$se.fit*1.96,rev(smoothed[[i]]$fit-smoothed[[i]]$se.fit*1.96)),
#    col=rgb(0.5,0.5,0.5,0.5),border=NA)
  lines(polls[,"Date"],pmax(0,smoothed[[i]]$fit),col=cols[i],lwd=2)   #Constraints it to be positive
}
matpoints(polls$Date,polls[,parties],pch=20,col=cols)
legend("topleft",legend=parties,col=cols,pch=20,bg="white",lwd=2)
axis(1,at=as.double(ticks),labels=format(ticks,format="%b\n%Y"),cex.axis=0.8)
axis(4,at=axTicks(4),labels=rep("",length(axTicks(4))))
#Add best estimates
for(i in 1:length(smoothed)) {
  lbl <- sprintf("%4.0f%% ± %2.0f",abs(round(rev(smoothed[[i]]$fit)[1],0)),round(1.96*rev(smoothed[[i]]$se.fit)[1],0))
  text(rev(polls$Date)[1],rev(smoothed[[i]]$fit)[1],labels=lbl,pos=4,col=cols[i])
}
dev.off()

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