How to use ggplot to align my two labels?

Question

I have a ggplot plot with two labels. I want to add both labels onto my plot, aligned vertically. I want my second label drawn below the first label, but they should not overlap.

So if I want to draw the first label at (20,90), where should the second label go? Obviously, it depends on the y-axis of the data set and the height required for the first label.

Q: The coordinates are hard-coded (see my script) and won't work for a new data set. I want to come up with a way such that the two labels are always aligned vertically for any data set and scaling. My potential solutions are:

1. Create a single label showing two lines, something like Line1\nLine2
2. Use a layout algorithm to force the alignment, like tableGrob from the gridExtra package.

I don't know how to make 1 works. In 2, I'm not sure how to make tableGrob works with individual labels. What's the easiest solution?

library(ggplot2)

data <- data.frame(x=c(1:100), y=x+rnorm(100))

p <- ggplot(data=data, aes_string(x='data$x', y='data$y')) + geom_point()

str1 <- "italic(y) == \"1.7\" + \"1\" * italic(x) * \",\" ~ ~italic(r)^2 ~ \"=\" ~ \"0.91\""
str2 <- "italic(y) == \"9.7\" + \"1\" * italic(x) * \",\" ~ ~italic(r)^2 ~ \"=\" ~ \"9.91\""

above <- annotate("text",
              label=str1,
              x=20,
              y=90,
              parse=TRUE)
below <- annotate("text",
              label=str2,
              x=20,
              y=83,
              parse=TRUE)

p <- p + above
p <- p + below

print(p)

Answer 1

You could use atop to make single label out of the two labels, stacked one atop the other:

str3 = 'atop(italic(y) == \"1.7\" + \"1\" * italic(x) * \",\" ~ ~italic(r)^2 ~ \"=\" ~ \"0.91\",
    italic(y) == \"9.7\" + \"1\" * italic(x) * \",\" ~ ~italic(r)^2 ~ \"=\" ~ \"9.91\")'

p + annotate("text",
               label=str3,
               x=20,
               y=90,
               parse=TRUE)

You do still need to enter the coordinates for the single label.

Answer 2

Try the package cowplot which gives you the function draw_label which lets you add layers with relative positioning:

library(ggplot2)
library(cowplot)

data <- data.frame(x = 1:100, y = (1:100) + rnorm(100))

p <- ggplot(data = data, 
            aes(x = x, 
                y = y)) + 
  geom_point()

ggdraw(p) + 
  draw_label(expression(paste(italic(y) == 1.7 + 1 * italic(x), 
                              ", ", 
                              italic(r)^2 == 0.91)), 
             .2, .9) +
  draw_label(expression(paste(italic(y) == 9.7 + 1 * italic(x), 
                              ", ", 
                              italic(r)^2 == 9.91)),
             .2, .83)

Answer 3

You could try something like this,

library(gridExtra)
library(grid)

table_label <- function(label, params=list())  {

  params <- modifyList(list(hjust=0, x=0), params)

  mytheme <- ttheme_minimal(padding=unit(c(1, 1), "mm"),core = list(fg_params = params), parse=TRUE)
  disect <- strsplit(label, "\\n")[[1]]
  m <- as.matrix(disect)
  tableGrob(m, theme=mytheme)

}

txt <- 'italic(y) == 1.7 + 1 * italic(x) * "," ~~italic(r)^2 ~ "=" ~ 0.91\n
        italic(y) == 9.7 + 1 * italic(x) * "," ~~italic(r)^2 ~ "=" ~ 9.91'


library(ggplot2)

qplot(1:10,1:10) +
  annotation_custom(table_label(txt), xmin=0, xmax=5, ymin=7.5)