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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)
library(tidyverse)

#==========================================
#Parameters - specified as a list
opts <- list()
opts$major <- list(parties= c("GRN", "LAB", "NAT", "NZF"),   #use precise names from Table headers
                   ylims = c(0,65),   #Vertical range
                   fname= "NZ_opinion_polls_2017-2020-majorparties.png",
                   dp=0)  #Number of decimal places to round estimates to
opts$minor <- list(parties=c("ACT","TOP", "MRI","NCP"   #please use "Maori" for the Maori party
                   ),
                   ylims = c(0,6),   #Vertical range
                   fname = "NZ_opinion_polls_2017-2020-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("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)
party.names <- gsub("<.*?>","",hdrs)[-c(1:3, 12)] %>% #nasty hack
  gsub(" ","_",.) %>% #Replace space with a _ 
  gsub("M.{1}ori","Maori",.) #Apologies, but the hard "a" is too hard to handle otherwise
  
#extract party colours
# party.cols <- str_extract(hdrs, "(?<=color:).{7}") %>%
#   na.omit()

party.cols <- c("#00529F", "#D82A20", "#000000", "#098137", 
                "#FDE401", "#800080", "#800000",   "#6698FF")
names(party.cols) <- party.names

names(party.cols) <- party.names

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

#Now extract the data
dat <- tbl.rows
dat <- lapply(dat, function(x) x[c(7:14)])
dat <- unlist(dat)
dat <- gsub("<.{1,3}>", "", dat)
dat <- gsub("<.*>", "", dat)
dat <- as.numeric(dat)
survey.dat <- matrix(dat, nrow = length(tbl.rows), byrow = TRUE) %>%
  as.data.frame()
names(survey.dat) = party.names

#get survey dates
date.str <- lapply(tbl.rows, function(x) x[2])
date.str <- str_extract(date.str, '".*"')
date.str <- gsub('\\"', '', date.str)
date.str <- strtrim(date.str, 10)

survey.date <- strptime(date.str, format = "%Y-%m-%d")

#get survey company
company <- lapply(tbl.rows, function(x) x[4]) %>%
  unlist()
company <- str_extract(company, '(?=">).*')
company <- gsub('\\">', '', company)
company <- strsplit(company, '<') %>%
  lapply(function(x) x[1]) %>%
  unlist()

#Combine results
surveys <- cbind(survey.date, company, survey.dat) %>%
  rename(Date = survey.date, Company = company) %>%
  filter(NAT > 0)



#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$Date)
  plt.dat <- subset(plt.dat,Company!="2017 election result")
  plt.dat$Company <- factor(plt.dat$Company)
  
  #Setup plot
  ticks <- ISOdate(c(rep(2017,1),rep(2018,2),rep(2019,2),rep(2020,2),2021),c(rep(c(7,1),4)),1)
  xlims <- range(c(ISOdate(2017,11,1),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)
  abline(v=1506121200, col = "black", lty = 1)
  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=0.5)
    smoothed[[i]] <- predict(smoother,newdata=predict.x,se=TRUE)
    poly.data <- data.frame(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))) %>%
      na.omit() %>%
      polygon(col=rgb(0.5,0.5,0.5,0.5),border=NA, fillOddEven = TRUE)
  }
  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")