library(OIdata)
library(RColorBrewer)
library(classInt)

# load state data from OIdata package:
data(state)

# set constants:
nclr <- 8 # number of bins
min <- 0 # theoretical minimum
max <- 100 # theoretical maximum
breaks <- (max - min) / nclr

# set up colors:
plotclr <- brewer.pal(nclr, "Oranges")
plotvar <- state$coal
class <- classIntervals(plotvar,
  nclr,
  style = "fixed",
  fixedBreaks = seq(min, max, breaks))
colcode <- findColours(class,
  plotclr)
NAColor <- "gray80"
plotclr <- c(plotclr, NAColor)

# map data:
map("state", # base
  col = NAColor,
  fill = TRUE,
  lty = 0)
map("state", # data
  col = colcode,
  fill = TRUE,
  lty = 0,
  add = TRUE)
map("state", # border
  col = "gray",
  lwd = 1.4,
  lty = 1,
  add = TRUE)

# set legend text:
legendText <- c()
for(i in seq(min, max - (max - min) / nclr, (max - min) / nclr)) {
  if (i == max(seq(min, max - (max - min) / nclr, (max - min) / nclr))) {
    legendText <- c(legendText, paste(round(i,3), "\u2264 n \u2264", round(i + (max - min) / nclr,3)))
    if (!is.na(NAColor)) legendText <- c(legendText, "NA")
  } else
    legendText <- c(legendText, paste(round(i,3), "\u2264 n <", round(i + (max - min) / nclr,3)))
}

legend("bottomleft", # position
  legend = legendText,
  title = "Percent",
  fill = plotclr,
  cex = 0.56,
  bty = "n") # border