Summary

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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.

rm(list=ls())
require(mgcv)
require(tidyverse)

#==========================================
#Parameters - specified as a list
opts <- list()
opts$major <- list(parties= c("Green","Labour","National","NZ First"),   #use precise names from Table headers
                   ylims = c(0,65),   #Vertical range
                   fname= "NZ_opinion_polls_2014-2017-majorparties.png",
                   dp=0)  #Number of decimal places to round estimates to
opts$minor <- list(parties=c("ACT","Maori","United","Mana","Con", "TOP"   #please use "Maori" for the Maori party
                   ),
                   ylims = c(0,6),   #Vertical range
                   fname = "NZ_opinion_polls_2014-2017-minorparties.png",
                   dp=1) #Number of decimal places to round estimates to

#==========================================
#Shouldn't need to edit anything below here
#==========================================

#Load the complete HTML file into memory
html <- readLines(url("https://en.wikipedia.org/wiki/Opinion_polling_for_the_New_Zealand_general_election,_2017",encoding="UTF-8"))


# html <- read_html("http://en.wikipedia.org/wiki/Opinion_polling_for_the_next_New_Zealand_general_election",encoding="UTF-8")
closeAllConnections()

#Extract the opinion poll data table
tbl.no <- 1
tbl <- html[(grep("<table.*",html)[tbl.no]):(grep("</table.*",html)[tbl.no])]

#Now split it into the rows, based on the <tr> tag
tbl.rows <- list()
open.tr <- grep("<tr",tbl)
close.tr <- grep("</tr",tbl)
for(i in 1:length(open.tr)) tbl.rows[[i]] <- tbl[open.tr[i]:close.tr[i]]

#Extract table headers
hdrs <- grep("<th",tbl,value=TRUE)
hdrs <- hdrs[1:(length(hdrs)/2 -10)]
party.names <- gsub("<.*?>","",hdrs)[-c(1:2)] %>% #nasty hack
  gsub(" ","_",.) %>% #Replace space with a _ 
  gsub("M.{1}ori","Maori",.) #Apologies, but the hard "a" is too hard to handle otherwise
  
# party.cols   <- gsub("^.*bgcolor=\"(.*?)\".*$","\\1",hdrs)[-c(1:2)]
party.cols <- c("#00529F", "#D82A20", "#098137", "#000000", "#EF4A42",
                "#FDE401", "#501557", "#00AEEF", "#770808", "#151A61")
names(party.cols) <- party.names

#Extract data rows
tbl.rows <- tbl.rows[sapply(tbl.rows,function(x) length(grep("<td",x)))>1]

###UGLY HACK
#party.names <- party.names[1:9]

#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 other than first two
  dat     <- td.tags[-c(1,2)]
  #Now strip the data and covert to numeric format
  dat     <- gsub("<td>|</td>|<b>|</b>|<td style=|background:#[0-9A-Z]{6}","",dat)
  dat     <- gsub("\"", "", dat)
  dat     <- gsub("%","",dat)
  dat     <- gsub("-","0",dat)
  dat     <- gsub("<|>","",dat)
  dat     <- as.numeric(dat)
  if(length(dat)!=length(party.names)) {
    stop(sprintf("Survey data is not defined properly: %s",td.tags[1]))
  }
  names(dat) <- party.names
  #Getting the date strings is a little harder. Start by tidying up the 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
  date.str <- gsub("<td>|</td>","",date.str)  #Throw out any tags
  #Get numeric parts of string
  digits.str <- gsub("[^0123456789]"," ",date.str)
  digits.str <- gsub("^ +","",digits.str)    #Drop leading whitespace
  digits     <- strsplit(digits.str," +")[[1]]
  yrs        <- grep("[0-9]{4}",digits,value=TRUE)
  days       <- digits[!digits%in%yrs]
  #Get months
  month.str <- gsub("[^A-Z,a-z]"," ",date.str)
  month.str <- gsub("^ +","",month.str)        #Drop leading whitespace
  mnths     <- strsplit(month.str," +",month.str)[[1]]
  #Now paste together to make standardised date strings
  days  <- rep(days,length.out=2)
  mnths <- rep(mnths,length.out=2)
  yrs <- rep(yrs,length.out=2)
  dates.std <- paste(days,mnths,yrs)
  #And finally the survey time
  survey.time <- mean(as.POSIXct(strptime(dates.std,format="%d %B %Y")))
  #Get the name of the survey company too
  survey.comp <- td.tags[1]
  survey.comp <- gsub("<sup.*</sup>","",survey.comp)
  survey.comp <- gsub("<td>|</td>","",survey.comp)
  survey.comp <- gsub("<U+2013>","-",survey.comp,fixed=TRUE)
  survey.comp <- gsub("(?U)<.*>","",survey.comp,perl=TRUE)
  survey.comp <- gsub("^ +| +$","",survey.comp)
  survey.comp <- gsub("-+"," ",survey.comp)
  
  #And now return results
  return(data.frame(Company=survey.comp,Date=survey.time,date.str,t(dat)))
})

#Combine results
surveys <- do.call(rbind,survey.dat)

##ugly date fix
surveys[26, 2] <- "2015-10-06 00:00:00"
surveys[29, 2] <- "2015-11-15 00:00:00"

#Ugly fix to remove Opportunities party while not enough data
# surveys <- select(surveys, -TOP)


#==========================================
#Now generate each plot
#==========================================


smoothers  <- list()
for(opt in opts) {
  
  #Restrict data to selected parties
  selected.parties <- gsub(" ","_",sort(opt$parties))
  selected.cols <- party.cols[selected.parties]
  plt.dat   <- surveys[,c("Company","Date",selected.parties)]
  plt.dat <- subset(plt.dat,!is.na(surveys$Date))
  plt.dat <- plt.dat[order(plt.dat$Date),]
  plt.dat$date.num  <- as.double(plt.dat$Date)
  plt.dat <- subset(plt.dat,Company!="2008 election result")
  plt.dat$Company <- factor(plt.dat$Company)
  
  #Setup plot
  ticks <- ISOdate(c(rep(2014,1),rep(2015,2),rep(2016,2),rep(2017,2),2018),c(rep(c(7,1),4)),1)
  xlims <- range(c(ISOdate(2014,11,1),ticks))
  png(opt$fname,width=778,height=487,pointsize=16)
  par(mar=c(5.5,4,1,1))
  matplot(plt.dat$date.num,plt.dat[,selected.parties],pch=NA,xlim=xlims,ylab="Party support (%)",
          xlab="",col=selected.cols,xaxt="n",ylim=opt$ylims,yaxs="i")
  abline(h=seq(0,95,by=5),col="lightgrey",lty=3)
  abline(v=as.double(ticks),col="lightgrey",lty=3)
  box()
  axis(1,at=as.double(ticks),labels=format(ticks,format="1 %b\n%Y"),cex.axis=0.8)
  axis(4,at=axTicks(4),labels=rep("",length(axTicks(4))))
  
  smoothed <- list()
  predict.x <- seq(min(surveys$Date),max(surveys$Date),length.out=100)
  for(i in 1:length(selected.parties)) {
    smoother <- loess(surveys[,selected.parties[i]] ~ as.numeric(surveys[,"Date"]),span=0.35)
    smoothed[[i]] <- predict(smoother,newdata=predict.x,se=TRUE)
    polygon(c(predict.x,rev(predict.x)),
            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)
  }
  names(smoothed) <- selected.parties
  #Then add the data points
  matpoints(surveys$Date, surveys[,selected.parties],pch=20,col=selected.cols)
  #And finally the smoothers themselves
  for(i in 1:length(selected.parties)) {
    lines(predict.x,smoothed[[i]]$fit,col=selected.cols[i],lwd=2)
  }
  
  # #Then add the data points
  # matpoints(plt.dat$date.num,plt.dat[,selected.parties],pch=20,col=selected.cols)
  # #And finally the smoothers themselves
  # for(n in selected.parties) {
  #   lines(smoothed.l[[n]]$date,smoothed.l[[n]]$fit,col=selected.cols[n],lwd=2)
  # }
  
  n.parties <- length(selected.parties)
  legend(grconvertX(0.5,"npc"),grconvertY(0.0,"ndc"),xjust=0.5,yjust=0,
         legend=gsub("_"," ",selected.parties), col=selected.cols,
         pch=20,bg="white",lwd=2,
         ncol=ifelse(n.parties>4,ceiling(n.parties/2),n.parties),xpd=NA)
  #Add best estimates
  fmt.str <- sprintf("%%2.%if\261%%1.%if %%%%",opt$dp,opt$dp)
  for(n in names(smoothed)) {
    lbl <- sprintf(fmt.str,
                   round(rev(smoothed[[n]]$fit)[1],opt$dp),
                   round(1.96*rev(smoothed[[n]]$se.fit)[1],opt$dp))
    text(rev(plt.dat$date.num)[1],rev(smoothed[[n]]$fit)[1],
         labels=lbl,pos=4,col=selected.cols[n],xpd=NA)
  }
  dev.off()
}

#==========================================
#Finished!
#==========================================

cat("Complete.\n")

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