r - ggplot2レーダーチャートから特定の線の色を変更

Question

質問には次の例を使用しました: http://www.cmap.polytechnique.fr/~lepennec/R/Radar/RadarAndParallelPlots.html

mtcarsscaled <- as.data.frame(lapply(mtcars, ggplot2:::rescale01))
mtcarsscaled$model <- rownames(mtcars)
mtcarsmelted <- reshape2::melt(mtcarsscaled)

coord_radar <- function (theta = "x", start = 0, direction = 1) 
{
  theta <- match.arg(theta, c("x", "y"))
  r <- if (theta == "x") 
    "y"
  else "x"
  ggproto("CordRadar", CoordPolar, theta = theta, r = r, start = start, 
          direction = sign(direction),
          is_linear = function(coord) TRUE)
}

plot <- ggplot(mtcarsmelted, aes(x = variable, y = value)) +
  geom_polygon(aes(group = model, color = model), fill = NA, size = 2, show.legend = FALSE) +
  geom_line(aes(group = model, color = model), size = 2) +
  theme(strip.text.x = element_text(size = rel(0.8)),
        axis.text.x = element_text(size = rel(0.8)),
        axis.ticks.y = element_blank(),
        axis.text.y = element_blank()) +
  xlab("") + ylab("") +
  guides(color = guide_legend(ncol=2)) +
  coord_radar()

print(plot)

プロット/グリッドの最も外側の線 (y ラベルの下の線) の色を変更するにはどうすればよいですか?

どんな助けでも大歓迎です！