r - 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)