如何在ggplot中独立定位两个图例

Question

标题很好地涵盖了它。

我有两个关于大小和颜色的图例，并希望有一个，比如说，在顶部，一个在图表中。

这可能吗，如果可以，怎么做

TIA

Answer 1

据我了解，基本上ggplot2对图例的控制非常有限。这是 Hadley 书中的一段（第 111 页）：

ggplot2 尝试使用尽可能少的图例来准确传达情节中使用的美学。如果变量与多个美学一起使用，它会通过组合图例来做到这一点。图 6.14 显示了点 geom 的一个例子：如果颜色和形状都映射到同一个变量，那么只需要一个图例。为了合并图例，它们必须具有相同的名称（相同的图例标题）。因此，如果您更改其中一个合并图例的名称，则需要为所有图例更改名称。

Answer 2

可以通过从图中提取单独的图例，然后将图例安排在相关图中来完成。此处的代码使用gtable 包中的函数进行提取，然后使用gridExtra 包中的函数进行排列。目的是要有一个包含颜色图例和尺寸图例的图。首先，从仅包含颜色图例的图中提取颜色图例。其次，从仅包含尺寸图例的图中提取尺寸图例。第三，绘制一个不包含图例的图。第四，将剧情和两个传说合并成一个新的剧情。

# Some data
df <- data.frame(
  x = 1:10,
  y = 1:10,
  colour = factor(sample(1:3, 10, replace = TRUE)),
  size = factor(sample(1:3, 10, replace = TRUE)))

library(ggplot2)
library(gridExtra)
library(gtable)
library(grid)

    ### Step 1
# Draw a plot with the colour legend
(p1 <- ggplot(data = df, aes(x=x, y=y)) +
   geom_point(aes(colour = colour)) +
   theme_bw() +
   theme(legend.position = "top"))

# Extract the colour legend - leg1
leg1 <- gtable_filter(ggplot_gtable(ggplot_build(p1)), "guide-box") 

    ### Step 2
# Draw a plot with the size legend
(p2 <- ggplot(data = df, aes(x=x, y=y)) +
   geom_point(aes(size = size)) +
   theme_bw())

# Extract the size legend - leg2
leg2 <- gtable_filter(ggplot_gtable(ggplot_build(p2)), "guide-box") 

    # Step 3
# Draw a plot with no legends - plot
(plot <- ggplot(data = df, aes(x=x, y=y)) +
   geom_point(aes(size = size, colour = colour)) +
   theme_bw() +
   theme(legend.position = "none"))

    ### Step 4
# Arrange the three components (plot, leg1, leg2)
# The two legends are positioned outside the plot: 
# one at the top and the other to the side.
plotNew <- arrangeGrob(leg1, plot, 
         heights = unit.c(leg1$height, unit(1, "npc") - leg1$height), ncol = 1)

plotNew <- arrangeGrob(plotNew, leg2,
          widths = unit.c(unit(1, "npc") - leg2$width, leg2$width), nrow = 1)

grid.newpage()
grid.draw(plotNew)

# OR, arrange one legend at the top and the other inside the plot.
plotNew <- plot + 
        annotation_custom(grob = leg2, xmin = 7, xmax = 10, ymin = 0, ymax = 4)

plotNew <- arrangeGrob(leg1, plotNew,
     heights = unit.c(leg1$height, unit(1, "npc") -  leg1$height), ncol = 1)

grid.newpage()
grid.draw(plotNew)

Answer 3

使用ggplot2和cowplot（= ggplot2 扩展）。

该方法类似于 Sandy 的方法，因为它将图例作为单独的对象取出，并让您独立进行放置。它主要是为多个图例设计的，这些图例属于图网格中的两个或多个图。

思路如下：

1. 创建 Plot1、Plot2、...、PlotX 没有图例
2. 使用图例创建 Plot1、Plot2、...、PlotX
3. 将第 1 步和第 2 步中的图例提取到单独的对象中
4. 设置图例网格并按照您想要的方式排列图例
5. 创建结合图和图例的网格

这似乎有点复杂且耗时/代码，但设置一次，您可以适应并使用它来进行各种情节/图例自定义。

library(ggplot2)
library(cowplot)

# Some data
df <- data.frame(
  Name = factor(rep(c("A", "B", "C"), 12)),
  Month = factor(rep(1:12, each = 3)),
  Temp = sample(0:40, 12),
  Precip = sample(50:400, 12)
)

# 1. create plot1
plot1 <- ggplot(df, aes(Month, Temp, fill = Name)) +
  geom_point(
    show.legend = F, aes(group = Name, colour = Name),
    size = 3, shape = 17
  ) +
  geom_smooth(
    method = "loess", se = F,
    aes(group = Name, colour = Name),
    show.legend = F, size = 0.5, linetype = "dashed"
  )

# 2. create plot2
plot2 <- ggplot(df, aes(Month, Precip, fill = Name)) +
  geom_bar(stat = "identity", position = "dodge", show.legend = F) +
  geom_smooth(
    method = "loess", se = F,
    aes(group = Name, colour = Name),
    show.legend = F, size = 1, linetype = "dashed"
  ) +
  scale_fill_grey()

# 3.1 create legend1
legend1 <- ggplot(df, aes(Month, Temp)) +
  geom_point(
    show.legend = T, aes(group = Name, colour = Name),
    size = 3, shape = 17
  ) +
  geom_smooth(
    method = "loess", se = F, aes(group = Name, colour = Name),
    show.legend = T, size = 0.5, linetype = "dashed"
  ) +
  labs(colour = "Station") +
  theme(
    legend.text = element_text(size = 8),
    legend.title = element_text(
      face = "italic",
      angle = -0, size = 10
    )
  )

# 3.2 create legend2
legend2 <- ggplot(df, aes(Month, Precip, fill = Name)) +
  geom_bar(stat = "identity", position = "dodge", show.legend = T) +
  scale_fill_grey() +
  guides(
    fill =
      guide_legend(
        title = "",
        title.theme = element_text(
          face = "italic",
          angle = -0, size = 10
        )
      )
  ) +
  theme(legend.text = element_text(size = 8))

# 3.3 extract "legends only" from ggplot object
legend1 <- get_legend(legend1)
legend2 <- get_legend(legend2)

# 4.1 setup legends grid
legend1_grid <- cowplot::plot_grid(legend1, align = "v", nrow = 2)

# 4.2 add second legend to grid, specifying its location
legends <- legend1_grid +
  ggplot2::annotation_custom(
    grob = legend2,
    xmin = 0.5, xmax = 0.5, ymin = 0.55, ymax = 0.55
  )

# 5. plot "plots" + "legends" (with legends in between plots)
cowplot::plot_grid(plot1, legends, plot2,
  ncol = 3,
  rel_widths = c(0.45, 0.1, 0.45)
)

^{由reprex package (v0.3.0) 于 2019 年 10 月 5 日创建}

---

更改最终plot_grid() 调用的顺序将图例移到右侧：

cowplot::plot_grid(plot1, plot2, legends, ncol = 3, 
                   rel_widths = c(0.45, 0.45, 0.1))