Add lines between specific levels in the legend in ggplot2

Question

I have a plot like this:

This is a mosaic plot where there is a black line above some groups. I would like this black line to be a legend too. In this example, the legend has 6 levels and above squares for levels 2 and 4, I would like a black line.

I tried something like: How to draw lines outside the plot area in ggplot2 , but, unfortunately, when I change the plot size, the lines move with the plot, not the legend, and they end up in the wrong place.

Here is an example of the code that made the plot above.

exampledata<-data.frame(var1Center=c(rep(.2, 6) ,rep(.5,6) ,rep(.8,6)), var2Height=c(.2,.2,.2,.1,.1,.2, .1,.1,.05,.45,.1,.2, .4,.07,.03,.1,.35,.05), var1=c(rep("Thing 1", 6), rep("Thing 2", 6), rep("Thing 3", 6)), var2=c( rep(c("A", "B", "C","D", "E", "F"), 3)), marginVar1=c(rep(.4,6) ,rep(.2,6), rep(.4,6))) plotlines<-data.frame(xstart=c(0, 0,.4, .4, .6,.6), xstop=c(.4,.4, .6,.6, 1,1), value=c(.4, .7, .2,.7, .47, .6)) ggplot(exampledata, aes(var1Center, var2Height)) + geom_bar(stat = "identity", aes(width = marginVar1, fill = var2)) + scale_x_continuous(breaks=exampledata$var1Center, labels=exampledata$var1, expand=c(0,0))+ theme_bw()+scale_y_continuous(name="Proportion",expand=c(0,0))+ guides(fill = guide_legend(reverse=TRUE))+ theme(panel.border=element_blank(), panel.grid=element_blank())+ theme(axis.text.x=element_text(angle=90, hjust=1, vjust=.3))+ geom_segment(data=plotlines, aes(x=xstart, xend=xstop, y=value, yend=value)) 

Answer 1

Here is the solution that the legendary grob edits. * I have added a longer answer below that shows how to study the grob structure to help find properties that you can edit.

* I just study grobs, so if someone has a decision on how to add linesGrob() to the legend, I would love to see it.

Short answer

 p1 <- ggplot()... #Your original plot gt <- ggplotGrob(pl) #Convert plot to grob library(gtable); library(gridExtra) leg <- gtable_filter(gt, "guide-box") #Extract the legend #Modify the legend by adding a black line that is horizontal y = c(1,1) #sub-grob 8 is the teal box D leg$grobs[[1]]$grobs[[1]]$grobs[[8]]$children[[2]]$gp$col <- "black" leg$grobs[[1]]$grobs[[1]]$grobs[[8]]$children[[2]]$y <- unit(c(1,1), "npc") #sub-grob 12 is the brown box B leg$grobs[[1]]$grobs[[1]]$grobs[[12]]$children[[2]]$gp$col <- "black" leg$grobs[[1]]$grobs[[1]]$grobs[[12]]$children[[2]]$y <- unit(c(1,1), "npc") #Plot first plot with no legend, then add modified legend. p2 <- grid.arrange(p1 +theme(legend.position = "none"), leg, ncol = 2, widths = unit.c(unit(1, "npc") - sum(leg$width), sum(leg$width))) 

Longer explanation The coffins are structured as lists, so use $ and [[]] to search for named and unnamed list items, respectively. The str command shows the structure of the object.

 str(leg) #List of 1 #$ grobs :List of 1 #..$ :List of 1 #.. ..$ grobs :List of 1 #.. .. ..$ 99_df28f764d4b38c6ac4aec87e00315c90:List of 20 #.. .. .. ..$ grobs :List of 20 #<--Here is where we can find the legend boxes #.. .. .. .. ..$ :List of 10 #.. .. .. .. .. ..$ x :Class 'unit' atomic [1:1] 0.5 #.. .. .. .. .. .. .. ..- attr(*, "unit")= chr "npc" #.. (snip) #'leg' has a named element 'grobs' with an unnamed list (' :List of 1'). #The first element of this unnamed list has a named sub-element 'grobs'. # which itself contains an unnamed list. #The next command goes down these branches of 'leg' to the sub-elements. leg$grobs[[1]]$grobs[[1]] #Elements of the legend are shown here #TableGrob (10 x 6) "layout": 20 grobs #z cells name grob #1 1 ( 1-10, 1- 6) background rect[legend.background.rect.171] #2 2 ( 2- 2, 2- 5) title text[guide.title.text.127] #3 3 ( 4- 4, 2- 2) key-3-1-bg rect[legend.key.rect.141] #4 4 ( 4- 4, 2- 2) key-3-1-1 gTree[GRID.gTree.142] #5 5 ( 5- 5, 2- 2) key-4-1-bg rect[legend.key.rect.146] #6 6 ( 5- 5, 2- 2) key-4-1-1 gTree[GRID.gTree.147] #7 7 ( 6- 6, 2- 2) key-5-1-bg rect[legend.key.rect.151] #8 8 ( 6- 6, 2- 2) key-5-1-1 gTree[GRID.gTree.152] #9 9 ( 7- 7, 2- 2) key-6-1-bg rect[legend.key.rect.156] #10 10 ( 7- 7, 2- 2) key-6-1-1 gTree[GRID.gTree.157] #11 11 ( 8- 8, 2- 2) key-7-1-bg rect[legend.key.rect.161] #12 12 ( 8- 8, 2- 2) key-7-1-1 gTree[GRID.gTree.162] #13 13 ( 9- 9, 2- 2) key-8-1-bg rect[legend.key.rect.166] #14 14 ( 9- 9, 2- 2) key-8-1-1 gTree[GRID.gTree.167] #15 15 ( 4- 4, 4- 4) label-3-3 text[guide.label.text.129] #16 16 ( 5- 5, 4- 4) label-4-3 text[guide.label.text.131] #17 17 ( 6- 6, 4- 4) label-5-3 text[guide.label.text.133] #18 18 ( 7- 7, 4- 4) label-6-3 text[guide.label.text.135] #19 19 ( 8- 8, 4- 4) label-7-3 text[guide.label.text.137] #20 20 ( 9- 9, 4- 4) label-8-3 text[guide.label.text.139] #This table shows the structure of the legened, # where cells indicate (min.X-max.X, min.Y-max.Y). #The 8th element is one of the color keys as a gTree grob. #Examining this legend key box in more detail: leg$grobs[[1]]$grobs[[1]]$grobs[[8]]$children #(rect[GRID.rect.153], lines[GRID.lines.154]) #'children' is composed 2 sub-elements (in an unnamed list): rectangle and lines. #Exploring second sub-element (lines): #Line properties str(leg$grobs[[1]]$grobs[[1]]$grobs[[8]]$children[[2]]) #List of 6 #$ x :Class 'unit' atomic [1:2] 0 1 #.. ..- attr(*, "unit")= chr "npc" #.. ..- attr(*, "valid.unit")= int 0 #$ y :Class 'unit' atomic [1:2] 0 1 #.. ..- attr(*, "unit")= chr "npc" #.. ..- attr(*, "valid.unit")= int 0 #$ arrow: NULL #$ name : chr "GRID.lines.154" #$ gp :List of 4 #..$ col : logi NA #..$ lwd : num 1.42 #..$ lineend: chr "butt" #..$ lty : num 1 #..- attr(*, "class")= chr "gpar" #$ vp : NULL #- attr(*, "class")= chr [1:3] "lines" "grob" "gDesc" #Here we see the line goes left-right with $x = c(0,1) and top bottom with $y = c(0,1). # ie a bottom-left to top-right diagonal line. #This line is not actually plotted because it has no color: $gp$col = NA grid.draw(leg) #Show the legend #y and gp$col are changed as noted above.