library(tidyverse)
library(htmltab)

election_graph <- function(pageURL) {
  election <- htmltab(pageURL,
                      which = 2,
                      rm_nodata_cols = F)
  election <- as.tibble(lapply(election, function(x) {gsub("unopp", "100", x)}))
  
  tidy_election <- gather(election, "Party", "Votes", 2:length(election))
  tidy_election$Year <- as.numeric(tidy_election$Year)
  tidy_election$Party <- factor(tidy_election$Party, levels = c("Conservative", "Labour", "Liberal", "Green", "SNP", "UKIP", "Other"))
  tidy_election$Votes <- as.numeric(tidy_election$Votes)
  
  election_victor <- tidy_election %>% filter(is.na(Votes) == FALSE) %>% group_by(Year) %>% summarize(Party = Party[which(Votes == max(Votes))])
  election_victor$Year <- as.numeric(election_victor$Year)
  election_victor$start_year <- election_victor$Year
  election_victor$end_year <- c(election_victor$Year[-1], ceiling(election_victor$Year[length(election_victor$Year)] + 1))
  election_victor[1,3] <- floor(election_victor[1,3] - 1)
  
  tidy_election$Votes <- as.numeric(sapply(tidy_election$Votes, function(x) {gsub(100, NA, x)}))
  
  party_colours <- c("#0087DC", "#DC241F", "#FAA61A", "#008066", "#FFF95D", "#EFE600", "dark grey")
  names(party_colours) <- c("Conservative", "Labour", "Liberal", "Green", "SNP", "UKIP", "Other")
  
  ggplot(tidy_election) +
    geom_rect(data = election_victor,
              aes(xmin = start_year,xmax = end_year, ymin = -Inf, ymax = Inf, fill = Party),
              alpha = 0.35,
              show.legend = F) +
    geom_line(aes(x = Year, y = Votes, colour = Party), size = 0.703) +
    geom_point(aes(x = Year, y = Votes, colour = Party)) +
    geom_hline(yintercept = 100, color="black", size = 1.5) +
    geom_vline(xintercept = 2019, color="black", size = 1.5) +
    scale_colour_manual(values = party_colours) +
    scale_fill_manual(values = party_colours) +
    theme(text = element_text(color="black", size = 14),
          axis.text = element_text(color="black", size = 11),
          axis.line.x = element_line(color="black", size = 0.703),
          axis.ticks.x = element_line(color="black", size = 0.703),
          axis.line.y = element_line(color="black", size = 0.703),
          axis.ticks.y = element_line(color="black", size = 0.703),
          axis.ticks.length = unit(5, "points"),
          panel.grid.major = element_line(color="blue", size = 0.5, linetype = 3),
          panel.grid.minor = element_blank(),
          panel.background = element_blank(),
          legend.position = c(.98, .97),
          legend.direction = "horizontal",
          legend.text = element_text(color="black", size = 11),
          legend.title=element_blank(),
          legend.justification = c("right", "top"),
          legend.box.just = "right",
          legend.key = element_blank(),
          legend.background = element_rect(fill = "white", colour = "black"),
          legend.margin = margin(0, 4, 4, 4)) +
    scale_x_continuous(expand = c(0, 0), limits = c(election_victor[[1,3]], election_victor$end_year[length(election_victor$end_year)]), breaks = seq(1890, 2010, 10)) +
    scale_y_continuous(expand = c(0, 0), limits = c(0, 100), breaks = seq(0, 100, 20)) +
    labs(x = "Year", y = "Percentage Vote")
}

election_graph("https://commons.wikimedia.org/wiki/File:HallamGraph.svg")

ggsave("HallamGraph.svg", device = "svg", units = "cm", width = 20, height = 11, dpi = 120)