Here is how to create the desired outcome:
library(reshape2); library(tidyverse)
melt(outer(1:4, 1:4), varnames = c("X1", "X2")) %>%
ggplot() +
geom_tile(aes(X1, X2, fill = value)) +
scale_fill_continuous(guide = guide_legend()) +
theme(legend.position="bottom",
legend.spacing.x = unit(0, 'cm'))+
guides(fill = guide_legend(label.position = "bottom"))
Created on 2019-12-07 by the reprex package (v0.3.0)
Edit: no need for these imperfect options anymore, but I'm leaving them here for reference.
Two imperfect options that don't give you exactly what you were asking for, but pretty close (will at least put the colours together).
library(reshape2); library(tidyverse)
df <- melt(outer(1:4, 1:4), varnames = c("X1", "X2"))
p1 <- ggplot(df, aes(X1, X2)) + geom_tile(aes(fill = value))
p1 + scale_fill_continuous(guide = guide_legend()) +
theme(legend.position="bottom", legend.direction="vertical")
p1 + scale_fill_continuous(guide = "colorbar") + theme(legend.position="bottom")
Created on 2019-02-28 by the reprex package (v0.2.1)