[R-sig-Geo] How to draw the same legend (one legend) for the multiple spatial figures?

[Bede-Fazekas Ákos]

Dear Brandon and Wei,
I don't know the answer to your question but a somewhat similar solution 
is when you plot a separate legend using lattice::draw.key() and disable 
sp::spplot()'s built-in legend using argument "auto.key = FALSE".
An example:

# loading libraries
library(lattice)
library(sp)
library(gridExtra)
library(grid)

# drawing legend
no_of_categories <- 10
cutpoints <- seq(from = 0 - 0.0001, to = 1 + 0.0001, length.out = 
no_of_categories + 1)
legend_text <- apply(X = cbind(round(cutpoints [1:no_of_categories], 1), 
round(cutpoints [2:(no_of_categories + 1)], 1)), MARGIN = 1, FUN = 
function(number) {return(paste(format(x = number, digits = 2), collapse 
= " - "))})
legend_colors <- colorRampPalette(c("red", "orange", "yellow", 
"lightgreen", "darkgreen"))(no_of_categories)
legend <- draw.key(key = list(reverse.rows = TRUE, space = "right", 
rectangles = list(col = legend_colors, border = FALSE), text = 
list(legend_text)), draw = FALSE)
dev.off()

# drawing maps
maps <- list()
for (column_name in colnames(sp_object at data)) {
     map <- spplot(obj = sp_object, zcol = column_name, auto.key = 
FALSE, col.regions = legend_colors, cuts = cutpoints)
     maps <- append(maps , list(map))
}
maps <- append(maps , list(legend))

# plotting the map-legend composite to a png file
layout <- rbind(c(1,2), c(3,4), c(5,6)) # let's say we have 5 maps and a 
legend
composite <- do.call(arrangeGrob, c(maps, list(layout_matrix = layout), 
list(widths = c(1, 1))))
png(width = 1000, height = 1000, filename = "map.png"))
     grid.newpage()
     grid.draw(composite)
dev.off()

Hope this helps,
Ákos Bede-Fazekas
Hungarian Academy of Sciences

2017.08.06. 11:53 keltezéssel, Brandon Payne írta:
> How to draw the same legend (one legend) for	the
>        multiple spatial figures?
>
> I would put the legend next to the upper-most plot and
> wrap the whole thing in a single
>
> /figure
>
> so that it would be more obvious that the same legend applied to all.
>
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