Evenly spaced dots ggplot2 demo

library(reshape2)
library(ggplot2)

df <- melt(volcano[20:44,10:34])

names(df) <- c("x","y","val")

rect <- data.frame(st = 5, en = 20)

ggplot() +
  geom_point(data = df,
    aes(x = x, y = y, color = val),
    size = 5) +
  geom_rect(data = rect,
    aes(xmin = st, xmax = en,
        ymin = st, ymax = en),
    color = "black",
    size = 2,
    fill = NA) +
  scale_color_gradient2(
    low = "blue", 
    mid = "white", 
    high = "red",
    limits = c(100, 200),
    midpoint = 150) +
  theme_bw()