Creating a half-donut, or parliamentary seating, chart

Question

I\'d like to create a chart displaying the size of the seats to a parliament, such as the one below, in ggplot2. My main problem is, essentially, how do I turn a donut chart to

Answer 1

To get labels etc you can use unit circle properties! I wrote a small function trying to recreate the style of the plot in your question :)

library(ggforce)

parlDiag <- function(Parties, shares, cols = NULL, repr=c("absolute", "proportion")) {
  repr = match.arg(repr)
  stopifnot(length(Parties) == length(shares))
  if (repr == "proportion") {
    stopifnot(sum(shares) == 1)
  }
  if (!is.null(cols)) {
    names(cols) <- Parties
  }

  # arc start/end in rads, last one reset bc rounding errors
  cc <- cumsum(c(-pi/2, switch(repr, "absolute" = (shares / sum(shares)) * pi, "proportion" = shares * pi)))
  cc[length(cc)] <- pi/2

  # get angle of arc midpoints
  meanAngles <- colMeans(rbind(cc[2:length(cc)], cc[1:length(cc)-1]))

  # unit circle
  labelX <- sin(meanAngles)
  labelY <- cos(meanAngles)

  # prevent bounding box < y=0
  labelY <- ifelse(labelY < 0.015, 0.015, labelY)

  p <- ggplot() + theme_no_axes() + coord_fixed() +
    expand_limits(x = c(-1.3, 1.3), y = c(0, 1.3)) + 
    theme(panel.border = element_blank()) +
    theme(legend.position = "none") +

    geom_arc_bar(aes(x0 = 0, y0 = 0, r0 = 0.5, r = 1,
                     start = cc[1:length(shares)], 
                     end = c(cc[2:length(shares)], pi/2), fill = Parties)) +

    switch(is.null(cols)+1, scale_fill_manual(values = cols), NULL) + 

    # for label and line positions, just scale sin & cos to get in and out of arc
    geom_path(aes(x = c(0.9 * labelX, 1.15 * labelX), y = c(0.9 * labelY, 1.15 * labelY),
                  group = rep(1:length(shares), 2)), colour = "white", size = 2) +
    geom_path(aes(x = c(0.9 * labelX, 1.15 * labelX), y = c(0.9 * labelY, 1.15 * labelY),
                  group = rep(1:length(shares), 2)), size = 1) +

    geom_label(aes(x = 1.15 * labelX, y = 1.15 * labelY, 
                   label = switch(repr,
                                  "absolute" = sprintf("%s\n%i", Parties, shares),
                                  "proportion" = sprintf("%s\n%i%%", Parties, round(shares*100)))), fontface = "bold", 
               label.padding = unit(1, "points")) +

    geom_point(aes(x = 0.9 * labelX, y = 0.9 * labelY), colour = "white", size = 2) +
    geom_point(aes(x = 0.9 * labelX, y = 0.9 * labelY)) +
    geom_text(aes(x = 0, y = 0, label = switch(repr, 
                                               "absolute" = (sprintf("Total: %i MPs", sum(shares))), 
                                               "proportion" = "")),
              fontface = "bold", size = 7) 

  return(p)
}

bt <- data.frame(parties = c("CDU", "CSU", "SPD", "AfD", "FDP", "Linke", "Grüne", "Fraktionslos"),
                 seats   = c(200, 46, 153, 92, 80, 69, 67, 2),
                 cols    = c("black", "blue", "red", "lightblue", "yellow", "purple", "green", "grey"),
                 stringsAsFactors = FALSE)

parlDiag(bt$parties, bt$seats, cols = bt$cols)