Question

I am trying to display some information about the data below the plot created in ggplot2. I would like to plot the N variable using the X axis coordinate of the plot but the Y coordinate needs to be 10% from the bottom of the screen . In fact, the desired Y coordinates are already in the data frame as y_pos variable.

I can think of 3 approaches using ggplot2:

1) Create an empty plot below the actual plot, use the same scale and then use geom_text to plot the data over the blank plot. This approach sort of works but is extremely complicated.

2) Use geom_text to plot the data but somehow use y coordinate as percent of the screen (10%). This would force the numbers to be displayed below the plot. I can’t figure out the proper syntax.

3) Use grid.text to display the text. I can easily set it at the 10% from the bottom of the screen but I can’t figure how set the X coordindate to match the plot. I tried to use grconvert to capture the initial X position but could not get that to work as well.

Below is the basic plot with the dummy data:

graphics.off()      # close graphics windows   

library(car)
library(ggplot2)  #load ggplot
library(gridExtra) #load Grid
library(RGraphics) # support of the "R graphics" book, on CRAN

#create dummy data
test= data.frame(
  Group = c("A", "B", "A","B", "A", "B"), 
  x = c(1 ,1,2,2,3,3 ),
  y = c(33,25,27,36,43,25),
  n=c(71,55,65,58,65,58),
  y_pos=c(9,6,9,6,9,6)
  )

#create ggplot
p1 <- qplot(x, y, data=test, colour=Group) +
  ylab("Mean change from baseline") + 
  geom_line()+
  scale_x_continuous("Weeks", breaks=seq(-1,3, by = 1) ) +
  opts( 
        legend.position=c(.1,0.9))

#display plot
p1

The modified gplot below displays numbers of subjects, however they are displayed WITHIN the plot. They force the Y scale to be extended. I would like to display these numbers BELOW the plot.

    p1 <- qplot(x, y, data=test, colour=Group) +
  ylab("Mean change from baseline") + 
  geom_line()+
  scale_x_continuous("Weeks", breaks=seq(-1,3, by = 1) ) +
  opts( plot.margin = unit(c(0,2,2,1), "lines"),
        legend.position=c(.1,0.9))+
  geom_text(data = test,aes(x=x,y=y_pos,label=n))

p1

A different approach of displaying the numbers involves creating a dummy plot below the actual plot. Here is the code:

graphics.off()      # close graphics windows   

library(car)
library(ggplot2)  #load ggplot
library(gridExtra) #load Grid
library(RGraphics) # support of the "R graphics" book, on CRAN

#create dummy data
test= data.frame(
  group = c("A", "B", "A","B", "A", "B"), 
  x = c(1 ,1,2,2,3,3 ),
  y = c(33,25,27,36,43,25),
  n=c(71,55,65,58,65,58),
  y_pos=c(15,6,15,6,15,6)
  )


p1 <- qplot(x, y, data=test, colour=group) +
  ylab("Mean change from baseline") + 
  opts(plot.margin = unit(c(1,2,-1,1), "lines")) +
  geom_line()+
  scale_x_continuous("Weeks", breaks=seq(-1,3, by = 1) ) +
  opts(legend.position="bottom",
       legend.title=theme_blank(),
       title.text="Line plot using GGPLOT") 
p1

p2 <- qplot(x, y, data=test, geom="blank")+
  ylab(" ")+
  opts(     plot.margin = unit(c(0,2,-2,1), "lines"),
            axis.line = theme_blank(), 
            axis.ticks = theme_segment(colour = "white"),           
            axis.text.x=theme_text(angle=-90,colour="white"),
            axis.text.y=theme_text(angle=-90,colour="white"),
            panel.background = theme_rect(fill = "transparent",colour = NA), 
            panel.grid.minor = theme_blank(),      
            panel.grid.major = theme_blank()
            )+
  geom_text(data = test,aes(x=x,y=y_pos,label=n)) 
p2

grid.arrange(p1, p2, heights = c(8.5, 1.5),             nrow=2 )

However, that is very complicated and would be hard to modify for different data. Ideally, I’d like to be able to pass Y coordinates as percent of the screen.

Answer 1

Edited opts has been deprecated, replaced by theme; element_blank has replaced theme_blank; and ggtitle() is used in place of opts(title = ...

Sandy- thank you so much!!!! This does exactly what I want. I do wish we could control the clipping in geom.text or geom.annotate.

I put together the following program if anybody else is interested.

 rm(list = ls())     # clear objects  
 graphics.off()      # close graphics windows   

library(ggplot2)
library(gridExtra)

#create dummy data
test= data.frame(
  group = c("Group 1", "Group 1", "Group 1","Group 2", "Group 2", "Group 2"), 
  x = c(1 ,2,3,1,2,3 ),
  y = c(33,25,27,36,23,25),
  n=c(71,55,65,58,65,58),
  ypos=c(18,18,18,17,17,17)

  )


p1 <- qplot(x=x, y=y, data=test, colour=group) +
  ylab("Mean change from baseline") + 
  theme(plot.margin = unit(c(1,3,8,1), "lines")) +
  geom_line()+
  scale_x_continuous("Visits", breaks=seq(-1,3) ) +
  theme(legend.position="bottom",
       legend.title=element_blank())+
   ggtitle("Line plot") 


# Create the textGrobs 
for (ii in 1:nrow(test))
{
  #display numbers at each visit
  p1=p1+ annotation_custom(grob = textGrob(test$n[ii]),  
                           xmin = test$x[ii], 
                           xmax = test$x[ii], 
                           ymin = test$ypos[ii], 
                           ymax = test$ypos[ii])

    #display group text
    if (ii %in% c(1,4)) #there is probably a better way 
      {
    p1=p1+ annotation_custom(grob = textGrob(test$group[ii]),  
                             xmin = 0.85, 
                             xmax = 0.85, 
                             ymin = test$ypos[ii], 
                             ymax = test$ypos[ii])
    }

  }




  # Code to override clipping
  gt <- ggplot_gtable(ggplot_build(p1))
  gt$layout$clip[gt$layout$name=="panel"] <- "off"
  grid.draw(gt)