如何在 ggplot2 的圖例中圍繞 alpha 值放置框

Question

我想在圖例中的 alpha 值周圍放置黑框，以便實際顯示最小值 (70)。 需要明確的是：圍繞代表每個 alpha 值的每個單獨的正方形，而不是圍繞整個圖例。

mtcars
test_plot <- ggplot() +
  geom_point(data = mtcars, 
             aes(x = wt, y = mpg,
                 alpha = disp,
                 size = hp), 
             fill = 'black',
             shape = 21) + 
  geom_point(data = mtcars, 
             aes(x = cyl, y = mpg,
                 size = hp), 
             fill = 'black',
             shape = 1) +
  xlab("Weight") + ylab("MPG") + 
  labs(size = "Horsepower:", alpha = "Displacement:") +
  scale_size_continuous(range = c(1,10), breaks= c(50, 100, 150, 200, 250, 300, 350)) +
  scale_alpha_continuous(range = c(0,1), 
                         breaks= c(71.1, 100, 200, 300, 400, 500), 
                         labels = c(70, 100, 200, 300, 400, 500)) +
  guides(size = guide_legend(override.aes = list(shape = 1)),
         alpha = guide_legend(override.aes = list(shape = 22, color = 'black')))

test_plot

plot 示例

Answer 1

問題是alpha值同時應用於圖例鍵的fill和邊框color 。 不幸的是，我不知道有一個簡單的解決方案。 獲得所需結果的一種方法是操作gtable 。

library(ggplot2)

p <- ggplot() +
  geom_point(data = mtcars, 
             aes(x = wt, y = mpg,
                 alpha = disp,
                 size = hp), 
             fill = 'black',
             shape = 21) + 
  geom_point(data = mtcars, 
             aes(x = cyl, y = mpg,
                 size = hp), 
             fill = 'black',
             shape = 1) +
  xlab("Weight") + ylab("MPG") + 
  labs(size = "Horsepower:", alpha = "Displacement:") +
  scale_size_continuous(range = c(1,10), breaks= c(50, 100, 150, 200, 250, 300, 350)) +
  scale_alpha_continuous(range = c(0,1), 
                         breaks= c(71.1, 100, 200, 300, 400, 500), 
                         labels = c(70, 100, 200, 300, 400, 500)) +
  guides(size = guide_legend(override.aes = list(shape = 1)),
         alpha = guide_legend(override.aes = list(shape = 22, color = 'black')))

library(gtable)
library(grid)

g <- ggplotGrob(p)
for (i in c(4, 6, 8, 10, 12)) { # Indices of legend keys
  # guide-box is element 15 in the gtable
  # alpha legend if the first element of the "guide-box"
  gp <- g$grobs[[15]]$grobs[[1]]$grobs[[i]]$gp
  gp <- modifyList(gp, list(col = "black"))
  
  g$grobs[[15]]$grobs[[1]]$grobs[[i]]$gp <- do.call(grid::gpar, gp)
}
grid.newpage()
grid.draw(g)

Answer 2

Map 變量fill而不是alpha ，具有一些具有邊緣的形狀，並使用scale_fill_gradient*和 colors 變化的 alpha。 使用stroke來控制邊框的大小。

我簡化了您的示例以突出代碼的相關點：

ggplot(mtcars, aes(wt, mpg)) +
  geom_point(aes(fill = disp), shape = 21, size = 10, stroke = 1e-10) +
  scale_fill_gradient(low = alpha("black", 0), high = alpha("black", 1),  guide = "legend") +
  guides(fill = guide_legend(override.aes = list(size = 6, stroke = .5)))