Function to build double y axis graph in ggplot2

Question

I am testing a function to build a double y axis graph in ggplot2. It works but I can't get some elements from the input graphics. I have built these two graphs with two data frames Base1 and Base2 (I add the dput() version in the final part):

library(ggplot2)
library(scales)
library(gtable)
#Graph 1
g1<-ggplot(Base1, aes(x = Month, y = value, fill = variable)) +
  geom_bar(stat="identity",colour="black",size=1) +
  scale_y_continuous(labels = comma,breaks=pretty_breaks(n=7),
                     limits=c(0,max(Base1$value,na.rm=T))) +
  theme(axis.text.x=element_text(angle=90,colour="grey20",face="bold",size=12),
        axis.text.y=element_text(colour="grey20",face="bold",hjust=1,vjust=0.8,size=15),
        axis.title.x=element_text(colour="grey20",face="bold",size=16),
        axis.title.y=element_text(colour="grey20",face="bold",size=16)) +
  xlab('Month')+ylab('')+ ggtitle("My graph") +
  theme(plot.title = element_text(lineheight=3, face="bold", color="black",size=24)) +
  theme(legend.text=element_text(size=14),
        legend.title=element_text(size=14)) +
  scale_fill_manual(name = "variable", 
                    label = "Power", 
                    values = "#FF6C91")  

And the second one:

#Graph2
colors=c("red","darkgreen")
g2<-ggplot(Base2, aes(x=Month, y=value, color=variable))+ 
  geom_line(aes(group=variable),size=1.3) +
  geom_point(size=3.8, shape=21, fill="white") + 
  scale_color_manual(values=colors)+ ggtitle("My graph")

With these two graphics I used the next function two make a double y axis graph:

double_axis_graph <- function(graf1,graf2){
  graf1 <- graf1
  
  graf2 <- graf2
  
  
  
  gtable1 <- ggplot_gtable(ggplot_build(graf1))
  
  gtable2 <- ggplot_gtable(ggplot_build(graf2))
  
  
  
  par <- c(subset(gtable1[['layout']], name=='panel', select=t:r))
  
  graf <- gtable_add_grob(gtable1, gtable2[['grobs']][[which(gtable2[['layout']][['name']]=='panel')]],
                          
                          par['t'],par['l'],par['b'],par['r'])
  
  
  
  ia <- which(gtable2[['layout']][['name']]=='axis-l')
  
  ga <- gtable2[['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')
  
  graf <- gtable_add_cols(graf, gtable2[['widths']][gtable2[['layout']][ia, ][['l']]], length(graf[['widths']])-1)
  
  graf <- gtable_add_grob(graf, ax, par['t'], length(graf[['widths']])-1, par['b'])
  
  return(graf)
  
}

So, when I used this to build the two axis graphs, the result shows the double axis, but I can't get the other elements from the input graphics as a complete legend; moreover, when I join the graphs only one of these is showed and the other is lost. I applied the function with the next results:

plot(double_axis_graph(g1,g2))

In this case, the bar graphic (g1) dissapears and I can't complete the legend with the elements from g2. Two axis worked fine. In a second test I got this result:

plot(double_axis_graph(g2,g1))

In this case I lost the series from g2 and the legend doesn't have the elements from g1. I would like to complete the function to show both graphics and the elements in the legend for all series in the graph.

data:

Base1 <- data.frame(
  Month = c("m11", "m12", "m13", "m14", "m15", "m16"),
  variable = factor(rep("Power", 6L)),
  value = c(28101696.45, 28606983.44, 30304944, 32583849.36, 34791542.82, 40051050.24)
)

Base2 <- data.frame(
  Month = rep(c("m11", "m12", "m13", "m14", "m15", "m16"), 2),
  variable = factor(rep(c("Index1", "Index2"), each = 6L)),
  value = c(
    0.044370892419913, 0.0437161234641523, 0.0516857394621815, 0.0495793011485982,
    0.0506456741259283, 0.0314653057147897, 0.0299405579124744,
    0.0296145768664101, 0.0269727649059507, 0.0250663815369419,
    0.0233469715385275, 0.0201801611981898
  )
)

Answer 1

First steps towards a solution:

g1.1 <- g1 + theme_bw() + theme(legend.position="top")
g2.1 <- g2 + theme_bw() + theme(panel.grid=element_blank()) +
    theme(panel.background = element_rect(fill = NA))
plot(double_axis_graph(g1.1,g2.1))

Now we need to fix:

• Weird grey background everywhere - edit: fixed it!
• The legend

And survive Hadley's wrath for doing what is fundamentally flawed.

Answer 2

I used some guys code to build my own here

grid.newpage()

# two plots
p1 <- ggplot(fuel, aes(x=date, y=mpg)) + geom_line() + 
  theme_bw() + xlab("Date") + ylab("MPG (Black)")
p2 <- ggplot(fuel, aes(x=date, y=avg.temp)) + 
  xlab("Date") + ylab("AVG Temp (Red)") +
  geom_line(colour="red") + theme_bw() + 
  theme(axis.title.y=element_text(angle = -90)) %+replace% 
  theme(panel.background = element_rect(fill = NA))

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

# overlap the panel of 2nd plot on that of 1st plot
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
alab <- g2$grobs[[which(g2$layout$name=="ylab")]]
ia <- which(g2$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$widths) - 1 )
g <- gtable_add_cols(g, g2$widths[g2$layout[ia, ]$l], 
                     length(g$widths) - 1 )
g <- gtable_add_grob(g, ax, pp$t, length(g$widths) - 2, pp$b)
g <- gtable_add_grob(g, alab, pp$t, length(g$widths) - 1, pp$b)

grid.draw(g)

Answer 3

Another smarter approach with sec_axis adapted from here.

Advantage:

1. Native secondary axis, only one ggplot object is created.
2. No hacks involving explicitly tweaking axis objects.

Disadvantage: Limits of the two axes must be defined beforehand.

Refer to sec_axis documentation and the blog post (in Japanese) for more information.

# You must define limits first

AXIS1_MIN = 0
AXIS1_MAX = max(Base1$value)
AXIS2_MIN = min(Base2$value)
AXIS2_MAX = max(Base2$value)

scale_to_value1 <- function(values) rescale(values, to = c(AXIS1_MIN, AXIS1_MAX))
scale_to_value2 <- function(values) rescale(values, to = c(AXIS2_MIN, AXIS2_MAX))

ggplot() +
  geom_bar(aes(x = Month, y = value, fill = variable), data = Base1, stat="identity",colour="black",size=1) +
  geom_line(aes(x = Month, y = scale_to_value1(value),color = variable, group = variable), data = Base2, size=1.3) +
  scale_y_continuous(labels = comma,breaks=pretty_breaks(n=7),
                     limits=c(AXIS1_MIN, AXIS1_MAX),
                     sec.axis = sec_axis( ~ scale_to_value2(.), name = "value2")) +
  scale_color_manual(name = "variable", 
                    label = c("Index1", "Index2"), 
                    values = c("Index1" = "red", "Index2" = "green")) +
  scale_fill_manual(name = "variable", 
                    label = "Power",
                    values = "#FF6C91") +
  theme(axis.text.x=element_text(angle=90,colour="grey20",face="bold",size=12),
        axis.text.y=element_text(colour="grey20",face="bold",hjust=1,vjust=0.8,size=15),
        axis.title.x=element_text(colour="grey20",face="bold",size=16),
        axis.title.y=element_text(colour="grey20",face="bold",size=16)) +
  xlab('Month')+ylab('')+ ggtitle("My graph") +
  theme(plot.title = element_text(lineheight=3, face="bold", color="black",size=24)) +
  theme(legend.text=element_text(size=14),
        legend.title=element_text(size=14))