Manipulating the legend of scale_fill_gradient2 - r

Question

I have data which comes from a statistical test (gene set enrichment analysis, but that's not important), so I obtain p-values for statistics that are normally distributed, i.e., both positive and negative values:
The test is run on several categories:
set.seed(1)
df <- data.frame(col = rep(1,7),
category = LETTERS[1:7],
stat.sign = sign(rnorm(7)),
p.value = runif(7, 0, 1),
stringsAsFactors = TRUE)
I want to present these data in a geom_tile ggplot such that I color code the df$category by their df$p.value multiplied by their df$stat.sign (i.e, the sign of the statistic)
For that I first take the log10 of df$p.value:
df$sig <- df$stat.sign*(-1*log10(df$p.value))
Then I order the df by df$sig for each sign of df$sig:
library(dplyr)
df <- rbind(dplyr::filter(df, sig < 0)[order(dplyr::filter(df, sig < 0)$sig), ],
dplyr::filter(df, sig > 0)[order(dplyr::filter(df, sig > 0)$sig), ])
And then I ggplot it:
library(ggplot2)
df$category <- factor(df$category, levels=df$category)
ggplot(data = df,
aes(x = col, y = category)) +
geom_tile(aes(fill=sig)) +
scale_fill_gradient2(low='darkblue', mid='white', high='darkred') +
theme_minimal() +
xlab("") + ylab("") + labs(fill="-log10(P-Value)") +
theme(axis.text.y = element_text(size=12, face="bold"),
axis.text.x = element_blank())
which gives me:
Is there a way to manipulate the legend such that the values of df$sig are represented by their absolute value but everything else remains unchanged? That way I still get both red and blue shades and maintain the order I want.

Answer 1

If you check ggplot's documentation, scale_fill_gradient2, like other continuous scales, accepts one of the following for its labels argument:
NULL for no labels
waiver() for the default labels computed for the transofrmation object
a character vector giving labels (must be same length as breaks)
a function that takes the breaks as input and returns labels as output
Since you only want the legend values to be absolute, I assume you're satisfied with the default breaks in the legend colour bar (-0.1 to 0.4 with increments in 0.1), so all you really need is to add a function that manipulates the labels.
I.e. instead of this:
scale_fill_gradient2(low = 'darkblue', mid = 'white', high = 'darkred') +
Use this:
scale_fill_gradient2(low = 'darkblue', mid = 'white', high = 'darkred',
labels = abs) +

Answer 2

I'm not sure I did understood what you're looking for. Do you meant that you wan't to change the labels within legends? If you want to change labels manipulating breaks and labels given by scale_fill_gradient2() shall do it.
ggplot(data=df,aes(x=col,y=category)) +
geom_tile(aes(fill=sig)) +
scale_fill_gradient2(low='darkblue',mid='white',high='darkred',
breaks = order(unique(df$sig)),
labels = abs(order(unique(df$sig)))) +
theme_minimal()+xlab("")+ylab("")+labs(fill="-log10(P-Value)") +
theme(axis.text.y=element_text(size=12,face="bold"),axis.text.x=element_blank())
For what you're looking for maybe you could display texts inside the figure to show the values, try stacking stat_bin_2d() like this:
ggplot(data=df,aes(x=col,y=category)) +
geom_tile(aes(fill=sig)) +
scale_fill_gradient2(low='darkblue',mid='white',high='darkred',
breaks = order(unique(df$sig)),
labels = abs(order(unique(df$sig)))) +
theme_minimal()+xlab("")+ylab("")+labs(fill="-log10(P-Value)") +
stat_bin_2d(geom = 'text', aes(label = sig), colour = 'black', size = 16) +
theme(axis.text.y=element_text(size=12,face="bold"),axis.text.x=element_blank())
You might want to give the size and colour arguments some tries.