How do I position two legends independently in ggplot

Question

Title pretty well covers it.

I have two legends, relating to size and colour, and wish to have one,say, on the top and one within the graph.

Is this possible and, if so, how

TIA

Romano

I don't think it is possible (where the legends are placed is controlled by the theme, and those parts which define legend location only take a single value). However, I'm not so sure that it is not possible to make this an answer. – Phrixus

Guess if it was, one of the experts would have been on by now – Romano

@pssguy, it can be done with some fiddling. In order to have control over the legends, you need to extract separate legends, then they can be arranged in a plot that initially contains no legend. – Hammer

legend.box.just in theme() was an adequate solution for me. – Gobang

Answer 1

From my understanding, basically there is very limited control over legends in ggplot2. Here is a paragraph from the Hadley's book (page 111):

ggplot2 tries to use the smallest possible number of legends that accurately conveys the aesthetics used in the plot. It does this by combining legends if a variable is used with more than one aesthetic. Figure 6.14 shows an example of this for the points geom: if both colour and shape are mapped to the same variable, then only a single legend is necessary. In order for legends to be merged, they must have the same name (the same legend title). For this reason, if you change the name of one of the merged legends, you’ll need to change it for all of them.

Luculent

Thanks for extract. All the examples show legends in same position – Romano

Answer 2

It can be done by extracting separate legends from plots, then arranging the legends in the relevant plot. The code here uses functions from the gtable package to do the extraction, then functions from the gridExtra package to do the arranging. The aim is to have a plot that contains a color legend and a size legend. First, extract the colour legend from a plot that contains the colour legend only. Second, extract the size legend from a plot that contains the size legend only. Third, draw a plot that contains no legend. Fourth, arrange the plot and the two legends into one new plot.

# 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)

Hammer

Hi Sandy. This is an impressive example. Can you break down the logic on the usage of the heights argument to arrangeGrob, for example in plotNew <- arrangeGrob(leg1, plot, heights = unit.c(leg1$height, unit(1, "npc") - leg1$height), ncol = 1)? I get that heights is passed as an argument to grid.layout, but I'm having trouble seeing how it is used here. Thanks. – Jerrold

Hi @Faheem , There are two plots to be arranged vertically: leg1 and plot; and therefore two heights in the unit.c() function. leg1 has an absolute height and is given by leg1$height. The second height, unit(1, "npc") - leg1$height, subtracts the legend height from the height of the device to give the available height for plot. In the first example, a similar logic applies to widths. There are two grobs to be plotted: the plot and the legend. Hence two widths in the unit.c() function; one for plotNew and the second for leg2. – Hammer

Thanks, Sandy. One more question - how do you know the height of the device is unit(1, "npc")? It would be useful if you added some of this info to your question. You could mention (a) the heights argument and widths arguments are passed to grid.layout, (b) that unit(1, "npc") is the height of the device, and therefore (c) e.g. leg1$height and unit(1, "npc") - leg1$height are the heights of the legend and the main plot respectively. I can add this if you want. – Jerrold

@Faheem, ?grid::unit gives details on units. Strictly, unit(1, "npc") is the height (or width) of the viewport. @baptiste, the author of the gridExtra package, has written about this sort of structure before on SO, also elsewhere but I can't locate it right now. Others (eg @agstudy) have used similar structures in their responses on SO. Thus, as for not including the detail in my post, I was assuming knowledge already at hand or knowledge that could be accessed easily. – Hammer

Hi sandy. Ok, I see that the page says "the viewport has a width and height of 1 unit.". I didn't know viewport referred to the device. Thanks. – Jerrold

I've posted a followup to this question: Positioning two legends independently in a ggplot2 plot. – Jerrold

Here is another followup of sorts, this GitHub issue:multiple calls to annotation_custom fail in certain cases. Sandy, I'd appreciate it if you can tell me whether you can reproduce. – Jerrold

@FaheemMitha I can reproduce your issue. Some time ago, I had a similar problem. Using annotation_custom I could position multiple tables, text, lines, rectangles, etc, but not multiple graphs. My workaround was to use viewports, adapting a response from https://mcmap.net/q/104042/-using-grconvertx-grconverty-in-ggplot2 – Hammer

Thanks, Sandy.I think it is better to fix bugs than work around them, but it can be difficult. I've done some analysis of the issue, though what I have focused on in the later parts may be an unrelated bug. – Jerrold

Is this still the best approach to achieving this? Just want to check as ~8 years have passed since the original answer and ~5 since the most recent edit – Maculation

Answer 3

Using ggplot2and cowplot (= ggplot2 extension).

The approach is similar to Sandy's one as it takes out the legend as seperate objects and lets you do the placement independently. It was primarly designed for multiple legends which belong to two or more plots in a grid of plots.

The idea is as follows:

1. Create Plot1, Plot2,...,PlotX without legends
2. Create Plot1, Plot2,...,PlotX with legends
3. Extract legends from step 1 & 2 into separate objects
4. Set up legend grid and arrange legends they way you want to
5. Create grid combining plots and legends

It seems kinda complicated and time/code consuming but set up once, you can adapt and use it for every kind of plot/legend customization.

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)
)

^{Created on 2019-10-05 by the reprex package (v0.3.0)}

---

Changing the order of the final plot_grid() call moves the legends to the right:

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

Gattis