how to show a legend on dual y-axis ggplot

问题

I am trying to compose a dual y-axis chart using ggplot. Firstly let me say that I am not looking for a discussion on the merits of whether or not it is good practice to do so. I find them to be particularly useful when looking at time based data to identify trends in 2 discrete variables. A further discussion of this is better suited to crossvalidated in my opinion.

Kohske provides a very good example of how to do it, which I have used to great effect so far. I am however at my limits to include a legend for both y-axes. I have also seen similar questions here and here but none seem to address the issue of including a legend.

I\'ve got a reproduceable example using the diamonds dataset from ggplot.

Data

library(ggplot2)
library(gtable)
library(grid)
library(data.table)
library(scales)

grid.newpage()

dt.diamonds <- as.data.table(diamonds)

d1 <- dt.diamonds[,list(revenue = sum(price),
                        stones = length(price)),
                  by=clarity]

setkey(d1, clarity)

Charts

p1 <- ggplot(d1, aes(x=clarity,y=revenue, fill=\"#4B92DB\")) +
  geom_bar(stat=\"identity\") +
  labs(x=\"clarity\", y=\"revenue\") +
  scale_fill_identity(name=\"\", guide=\"legend\", labels=c(\"Revenue\")) +
  scale_y_continuous(labels=dollar, expand=c(0,0)) + 
  theme(axis.text.x = element_text(angle = 90, hjust = 1),
        axis.text.y = element_text(colour=\"#4B92DB\"), 
        legend.position=\"bottom\")

p2 <- ggplot(d1, aes(x=clarity, y=stones, colour=\"red\")) +
  geom_point(size=6) + 
  labs(x=\"\", y=\"number of stones\") + expand_limits(y=0) +
  scale_y_continuous(labels=comma, expand=c(0,0)) +
  scale_colour_manual(name = \'\',values =c(\"red\",\"green\"), labels = c(\"Number of Stones\"))+
  theme(axis.text.y = element_text(colour = \"red\")) +
  theme(panel.background = element_rect(fill = NA),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank(),
        panel.border = element_rect(fill=NA,colour=\"grey50\"),
        legend.position=\"bottom\")

# extract gtable
g1 <- ggplot_gtable(ggplot_build(p1))
g2 <- ggplot_gtable(ggplot_build(p2))


pp <- c(subset(g1$layout, name == \"panel\", se = t:r))
g <- gtable_add_grob(g1, g2$grobs[[which(g2$layout$name == \"panel\")]], pp$t,
                     pp$l, pp$b, pp$l)
# axis tweaks
ia <- which(g2$layout$name == \"axis-l\")
ga <- g2$grobs[[ia]]
ax <- ga$children[[2]]
ax$widths <- rev(ax$widths)
ax$grobs <- rev(ax$grobs)
ax$grobs[[1]]$x <- ax$grobs[[1]]$x - unit(1, \"npc\") + unit(0.15, \"cm\")
g <- gtable_add_cols(g, g2$widths[g2$layout[ia, ]$l], length(g$widths) - 1)
g <- gtable_add_grob(g, ax, pp$t, length(g$widths) - 1, pp$b)
# draw it
grid.draw(g)

QUESTION: Does anyone have some tips on how to get the 2nd part of the legend to show?

The following are the charts produced in order p1, p2, combined p1&p2, you\'ll notice that the legend for p2 doesn\'t show in the combined chart.

$\"p1\"$

$\"p2\"$

combined p1 & p2

$\"combined$

回答1:

Similar to the technique you use above you can extract the legends, bind them and then overwrite the plot legend with them.

So starting from # draw it in your code

# extract legend
leg1 <- g1$grobs[[which(g1$layout$name == "guide-box")]]
leg2 <- g2$grobs[[which(g2$layout$name == "guide-box")]]

g$grobs[[which(g$layout$name == "guide-box")]] <- 
                                  gtable:::cbind_gtable(leg1, leg2, "first")
grid.draw(g)

来源：https://stackoverflow.com/questions/26727741/how-to-show-a-legend-on-dual-y-axis-ggplot

标签

charts

ggplot2

data.table

gtable