How to add bounding box to a specific area in ggplot2 heatmap?

Question

I use an example from here. My question is how can I add a specific bounding box to this heatmap, such as add a red line box to the top left four tiles?

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Answer 1

A quick and dirty (some hard-coding) possibility is to use geom_rect, where the positions are given by the numerical values of the levels of x and y variables to be bound with a box, plus/minus an offset.

ggplot(mymelt, aes(x = industry, y = variable, fill = value, label = value)) +
  geom_tile() +
  geom_text() +
  geom_rect(aes(xmin = 1 - 0.5, xmax = 2 + 0.5, ymin = 2 - 0.5, ymax = 3 + 0.5),
               fill = "transparent", color = "red", size = 1.5)

A less hard-coded version:

# convert x and y variables to factors
ind <- as.factor(mymelt$industry)
vars <- as.factor(mymelt$variable)

# numeric version of the levels to be bound by a box
xmin <- unique(as.numeric(ind[ind == "all industries"]))
xmax <- unique(as.numeric(ind[ind == "cars"]))

ymin <- unique(as.numeric(vars[vars == "americas"]))
ymax <- unique(as.numeric(vars[vars == "europe"]))

# set offset
offset <- 0.5

ggplot(mymelt, aes(x = industry, y = variable, fill = value, label = value)) +
  geom_tile() +
  geom_text() +
  geom_rect(aes(xmin = xmin - offset,
                xmax = xmax + offset,
                ymin = ymin - offset,
                ymax = ymax + offset),
            fill = "transparent", color = "red", size = 1.5)