Get width of plot area in ggplot2

Question

Is there any way to get the width of the plot area in the grid window? It grows or shrinks, for instance, if plot.margin is changed or if the y-axis labels' font-size is increased. Is is hidden somewhere in str(p)?

Any size measure would work. I need to be able to measure the relative change in the width of the plot area in different scenarios such as change of y-axis labels' font-size.

df = data.frame(x = (1:3),One=c(12, 8, 13),Two=c(13, 7, 11),Three=c(11, 9, 11))
df.melt = melt(df, id.vars="x")

p = ggplot(df.melt, aes(x=x, y=value, color=variable)) + 
  geom_line() +
  coord_cartesian(xlim=c(min(df.melt$x),max(df.melt$x))) +
  theme(legend.position="none", plot.margin = unit(c(1, 4, 1, 1), "cm")) 
p

---

UPDATE – To clarify: Please help me calculate a/b.

p = ggplot(df.melt, aes(x=x, y=value, color=variable)) + 
  geom_line() + coord_cartesian(xlim=c(min(df.melt$x),max(df.melt$x))) +
  theme(legend.position="none")

p1 = p + theme(plot.margin=unit(c(1,1,1,1),"cm"), axis.text.y=element_text(size=10))
p2 = p + theme(plot.margin=unit(c(1,1,1,2),"cm"), axis.text.y=element_text(size=30))
grid.arrange(p1, p2, ncol=2)

Answer 1

The plot in ggplot2 uses grid graphics. A graphical scene that has been produced using the grid graphics package consists of grobs and viewports.

You can use gridDebug package for the inspection of the grobs.

1. showGrob show the locations and names of the grobs used to draw the scene

         showGrob()
   

2. Get the gpath of the grob

     sceneListing <- grid.ls(viewports=T, print=FALSE)
     do.call("cbind", sceneListing)
   
      name                                gPath                                                      
   [1,] "ROOT"                              ""                                                         
   [2,] "GRID.gTableParent.45019"           ""                                                         
   [3,] "background.1-5-6-1"                "GRID.gTableParent.45019"                                  
   [4,] "spacer.4-3-4-3"                    "GRID.gTableParent.45019"                                  
   [5,] "panel.3-4-3-4"                     "GRID.gTableParent.45019"                                  
   [6,] "grill.gTree.44997"                 "GRID.gTableParent.45019::panel.3-4-3-4"                   
   

3. Retrieve the gorb

   h <- grid.get(gPath="GRID.gTableParent.45019")
   

4. get h properties (e.g)

   h$layoutvp$width
   

Application:

grid.get('x',grep=TRUE,global=T)
(polyline[panel.grid.minor.x.polyline.21899], polyline[panel.grid.major.x.polyline.21903], gTableChild[axis-l.3-3-3-3], gTableChild[axis-b.4-4-4-4], gTableChild[xlab.5-4-5-4]) 
>  grid.get('x',grep=TRUE,global=T)[[3]]
gTableChild[axis-l.3-3-3-3] 
>  xx <- grid.get('x',grep=TRUE,global=T)[[3]]
> grobWidth(xx)
[1] sum(1grobwidth, 0.15cm+0.1cm)

Answer 2

This intrigued me enough to look into it deeper. I was hoping that the grid.ls function would give the information to navigate to the correct viewports to get the information, but for your example there are a bunch of the steps that get replaced with '...' and I could not see how to change that to give something that is easily worked with. However using grid.ls or other tools you can see the names of the different viewports. The viewports of interest are both named 'panel.3-4-3-4' for your example, below is some code that will navigate to the 1st, find the width in inches, navigate to the second and find the width of that one in inches.

grid.ls(view=TRUE,grob=FALSE)
current.vpTree()
seekViewport('panel.3-4-3-4')
a <- convertWidth(unit(1,'npc'), 'inch', TRUE)
popViewport(1)
seekViewport('panel.3-4-3-4')
b <- convertWidth(unit(1,'npc'), 'inch', TRUE)
a/b

I could not figure out an easy way to get to the second panel without poping the first one. This works and gives the information that you need, unfortunately since it pops the 1st panel off the list you cannot go back to it and find additional information or modify it. But this does give the info you asked for that could be used in future plots.

Maybe someone else knows how to navigate to the second panel without popping the first, or getting the full vpPath of each of them to navigate directly.