library(reshape2)
library(ggplot2)

m <- matrix(1:64 - 32, 8)
rownames(m) <- colnames(m) <-
  c(paste0("a", 1:3), paste0("b", 1:2), paste0("c", 1:3))
d <- melt(m)
gg <- ggplot(d) +
  geom_tile(aes(x = Var1, y = Var2, fill = value)) +
  scale_fill_gradient2()

d$group <- substr(d$Var1, 1, 1)

xmin <- c(0.5,3.5,5.5)
xmax <- c(3.5,5.5,8.5)
ymin <- c(0.5,3.5,5.5)
ymax <- c(3.5,5.5,8.5)
box1 <- data.frame(xmin = rep(xmin,times = 3),
                   xmax = rep(xmax,each = 3),
                   ymin = rep(ymin,times = 3),
                   ymax = rep(ymax,each = 3))
ggplot(melt(m)) +
    geom_tile(aes(x = Var1, y = Var2, fill = value)) +
    geom_rect(data = box1,aes(xmin = xmin,xmax = xmax,ymin = ymin,ymax= ymax),
                        fill = NA,color = "black") +
    scale_fill_gradient2()

library('reshape2')
library('ggplot2')

m <- matrix(1:64 - 32, 8)
rownames(m) <- colnames(m) <-
  c(paste0("a", 1:3), paste0("b", 1:2), paste0("c", 1:3))
gg <- ggplot(melt(m)) +
  geom_tile(aes(x = Var1, y = Var2, fill = value)) +
  scale_fill_gradient2()

tt <- table(gsub('\d+', '', colnames(m)))
ll <- unname(c(0, cumsum(tt)) + .5)

gg + geom_segment(aes(x = ll, xend = ll, y = head(ll, 1), yend = tail(ll, 1))) +
  geom_segment(aes(y = ll, yend = ll, x = head(ll, 1), xend = tail(ll, 1)))