This chart was created with R.

This graph was created using R's ggplot2 package. Here is the source code, available under GPL version 2 and later as well as the licenses below.

library(reshape)
library(ggplot2)
polls <- read.csv("NZ opinion polls 2006-2008.csv")
polls$Date <- as.Date(polls$Date)
lpolls <- melt(polls[,-3], id.var=c("Poll", "Date"), variable_name="Party")
names(lpolls)[4] <- "Support"
levels(lpolls$Party)[3:4] <- c("NZ First", "Maori Party")
levels(lpolls$Party)[7] <- "United Future"
lpolls$Party <- with(lpolls, reorder(Party, Support, na.rm=TRUE))
p <- ggplot(lpolls[(lpolls$Date >= as.Date("2005-09-17")) & 
    (lpolls$Party %in% c("Labour", "National", "NZ First", "Greens")),], 
    aes(x=Date, y=Support, colour=factor(Party)))
p + geom_point() + stat_smooth() + 
    scale_colour_manual(name = "Party", values = c("black", "forestgreen", "red", "blue")) + 
    scale_x_date(major="4 months", format="%b-%Y") +
    scale_y_continuous(breaks=seq(from=0, to=55, by=5), name="Party support (%)")

I, the copyright holder of this work, hereby publish it under the following licenses:

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.http://www.gnu.org/copyleft/fdl.htmlGFDLGNU Free Documentation Licensetruetrue

This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

You are free:

• to share – to copy, distribute and transmit the work
• to remix – to adapt the work

Under the following conditions:

• attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
• share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.

https://creativecommons.org/licenses/by-sa/4.0CC BY-SA 4.0 Creative Commons Attribution-Share Alike 4.0 truetrue

You may select the license of your choice.