legend_cont: Add a continuous legend

In terra: Spatial Data Analysis

Add a continuous legend

Description

Add a continuous (color gradient) legend to an existing plot. This can be used with any base plot, not just plot,SpatRaster,numeric-method. The legend is drawn as a color bar with tick marks and labels.

Usage

legend_cont(x="right", y=NULL, legend, col,
    size=NULL, title=NULL, at=NULL, digits=NULL, ...)

Arguments

x	character or numeric. Position of the legend. Keywords: "right", "left", "top", "bottom", "topright", "bottomright". Or a numeric x-coordinate (see y)
y	numeric. Optional y-coordinate(s) for the legend position, used when x is numeric
legend	SpatRaster or numeric vector. Used to determine the value range of the legend. If a SpatRaster, the range is taken from minmax. If numeric (e.g. c(min, max)), the range of the values is used
col	character. Vector of colors for the gradient. If missing, the default palette is used (map.pal("viridis", 100))
size	numeric. One or two values to control the size of the legend bar. The first value scales the length (0 to 1), the second scales the width
title	character. Title to display above or beside the legend bar
at	numeric. Specific values at which to place tick marks. If NULL (the default), ticks are placed automatically with pretty
digits	non-negative integer. Number of decimal places for tick labels. If NULL (the default) an appropriate number is computed from the range
...	Additional legend parameters: bg (background color behind the legend, e.g. "white"), cex, horiz, reverse, labels, tic (or tick), tic.col, tic.lwd, tic.box.col, and others (see the plg argument in plot,SpatRaster,numeric-method for details)

Value

Invisible list with the legend parameters (can be used for further customization).

See Also

add_legend, add_box, plot,SpatRaster,numeric-method

Examples

# with a SpatRaster
r <- rast(ncols=40, nrows=40, xmin=0, xmax=1, ymin=0, ymax=1, vals=runif(1600))
plot(r, legend=FALSE, mar=c(3.1, 3.1, 2.1, 7.1))
legend_cont("right", legend=r)

# horizontal
plot(r, legend=FALSE, mar=c(6.1, 3.1, 2.1, 2.1))
legend_cont("bottom", legend=r, horiz=TRUE, title="values")

# on the map 
plot(r, legend=FALSE, alpha=0.1)
legend_cont(0.1, legend=r)
legend_cont(0.3, c(0.5, 0.9), legend=r, bg="white")
legend_cont(c(0.4, 0.8), 0.2, horiz=TRUE, legend=r, bg="white")


# with a numeric range and custom colors
cols <- heat.colors(100)
vals <- seq(5,95,10)
par(mar=c(3.1, 3.1, 2.1, 7.1))
plot(1:10, col=cols[vals], cex=2, pch=20)
legend_cont("right", legend=c(1, 100), col=cols)

terra documentation built on Aug. 22, 2026, 5:07 p.m.