ggplot2 geom_tile diagonal line overlay - r

Question

I'm looking for a way to produce a diagonal slash from the bottom left the to top right corner of a cell within a plot made using geom_tile.
The input is a melted data frame with two categorical factor columns, sample and gene. I'd like to use something like geom_segment, but I'm not able to specify fractional increments. Any ideas on the best way to accomplish this?
edit: Here is a reproducible example, I can't share one from my own data, as it's protected patient information.
df <- data_frame( gene = c('TP53','TP53','MTOR','BRACA1'),
sample = c('A','B','A','B'),
diagonal = c(FALSE,TRUE,TRUE,FALSE),
effect = c('missense', 'nonsense', 'missense', 'silent') )
ggplot(df, aes(sample, gene)) + geom_tile(aes(fill = effect))
what I'm looking for:

Answer 1

One way to do it:
library(ggplot2)
df <- data.frame(
x = rep(c(2, 5, 7, 9, 12), 2),
y = rep(c(1, 2), each = 5),
z = factor(1:10),
w = rep(diff(c(0, 4, 6, 8, 10, 14)), 2)
)
p <- ggplot(df, aes(x, y)) + geom_tile(aes(fill = z))
gb <- ggplot_build(p)
p + geom_segment(data=gb$data[[1]][1:2, ],
aes(x=xmin, xend=xmax, y=ymin, yend=ymax),
color="white")
In your example, could also rely on the indices of the factor levels like this:
library(ggplot2)
df <- data.frame( gene = c('TP53','TP53','MTOR','BRACA1'),
sample = c('A','B','A','B'),
diagonal = c(FALSE,TRUE,TRUE,FALSE),
effect = c('missense', 'nonsense', 'missense', 'silent') )
df$cross <- c(F,T,T,F)
ggplot(df, aes(sample, gene)) +
geom_tile(aes(fill = effect)) +
geom_segment(data=transform(subset(df, !!cross), sample=as.numeric(sample), gene=as.numeric(gene)),
aes(x=sample-.49, xend=sample+.49, y=gene-.49, yend=gene+.49),
color="white", size=2)
(Note that I used data.frame and not dplyr::data_frame, so that both columns become factors.)
If you want a legend:
ggplot(df, aes(sample, gene)) +
geom_tile(aes(fill = effect)) +
geom_segment(data=transform(subset(df, !!cross), sample=as.numeric(sample), gene=as.numeric(gene)),
aes(x=sample-.49, xend=sample+.49, y=gene-.49, yend=gene+.49, color=cross),
size=2) +
scale_color_manual(values=c("TRUE"="white", "FALSE"=NA))

Answer 2

You can use geom_abline. You can tweak intercept and slope to get what you want. More info and examples here.
ggplot(df, aes(sample, gene)) +
geom_tile(aes(fill = effect)) +
geom_abline(intercept = 1, slope = 1, color="white", size=2)

Answer 3

If you don't actually want specific lines, but just want to highlight, you can simply draw dots:
ggplot(df, aes(sample, gene)) + geom_tile(aes(fill = effect)) +
geom_point(aes(sample, gene))
You can make it look like a line: geom_point(aes(sample, gene), shape='/', size=10, color='white')
To have the lines be only on some tiles, simply pass only the rows with those coordinates to geom_point: geom_point(data=filter(df, diagonal), aes(sample, gene))
Alternatively, you can hack it with a manual shape scale: geom_point(aes(sample, gene, shape=diagonal)) + scale_shape_manual(values=c(' ', '/'))