Add a continuous color scale legend to an standard R plot

splot() is designed to combine a standard R plot with a legend representing a (continuous) color scale. This is done by splitting the plotting region into two parts. Keeping one for the main chart and putting the legend in the other.

Usage

splot(
  slim = c(0, 1),
  col = jet.colors(128),
  breaks = NULL,
  horizontal = FALSE,
  legend.shrink = 0.9,
  legend.width = 1.2,
  legend.mar = ifelse(horizontal, 3.1, 5.1),
  legend.lab = NULL,
  bigplot = NULL,
  smallplot = NULL,
  lab.breaks = NULL,
  axis.args = NULL,
  legend.args = NULL,
  add = FALSE
)

Arguments

slim: limits used to set up the color scale.
col: color table used to set up the color scale (see image for details).
breaks: (optional) numeric vector with the breakpoints for the color scale: must have one more breakpoint than col and be in increasing order.
horizontal: logical; if FALSE (default) legend will appear on the right side. If TRUE the legend will be along the bottom.
legend.shrink: amount to shrink the size of legend relative to the full height or width of the plot.
legend.width: width in characters of the legend strip. Default is 1.2, a little bigger that the width of a character.
legend.mar: width in characters of legend margin that has the axis. Default is 5.1 for a vertical legend and 3.1 for a horizontal legend.
legend.lab: label for the axis of the color legend. Default is no label as this is usual evident from the plot title.
bigplot: plot coordinates for main plot. If not passed these will be determined within the function.
smallplot: plot coordinates for legend strip. If not passed these will be determined within the function.
lab.breaks: if breaks are supplied these are text string labels to put at each break value. This is intended to label axis on a transformed scale such as logs.
axis.args: additional arguments for the axis function used to create the legend axis (see image.plot for details).
legend.args: arguments for a complete specification of the legend label. This is in the form of list and is just passed to the mtext function. Usually this will not be needed (see image.plot for details).
add: logical; if TRUE the legend is just added to the existing plot (the graphical parameters are not changed).

Value

Invisibly returns a list with the following 3 components:

bigplot: plot coordinates of the main plot. These values may be useful for drawing a plot without the legend that is the same size as the plots with legends.
smallplot: plot coordinates of the secondary plot (legend strip).
old.par: previous graphical parameters (par(old.par) will reset plot parameters to the values before entering the function).

Details

For instance, sxxxx() functions (spoints(), simage() and spersp()) draw the corresponding high-level plot (xxxx()), after calling splot(), to include a legend strip for the color scale.

These functions are based on function image.plot of package fields, see its documentation for additional information.

Side Effects

After exiting splot, the plotting region may be changed (par("plt")) to make it possible to add more features to the plot.

Author

Modified by Ruben Fernandez-Casal rubenfcasal@gmail.com.

Examples

# Plot equivalent to spoints():
scale.range <- range(mtcars$mpg)
res <- splot(slim = scale.range, legend.lab = "mpg")
with( mtcars, plot(hp, qsec, col = scolor(mpg, slim = scale.range),
      pch = 16, cex = 1.5, main = "Motor Trend Car Road Tests"))

par(res$old.par) # restore graphical parameters
# Multiple plots with a common legend:
# regularly spaced 2D data...
set.seed(1)
nx <- c(40, 40) # ndata =  prod(nx)
x1 <- seq(-1, 1, length.out = nx[1])
x2 <- seq(-1, 1, length.out = nx[2])
trend <- outer(x1, x2, function(x,y) x^2 - y^2)
y <- trend + rnorm(prod(nx), 0, 0.1)
scale.range <- c(-1.2, 1.2)
scale.color <- jet.colors(256)
# 1x2 plot with some room for the legend...
old.par <- par(mfrow = c(1,2), omd = c(0.05, 0.85, 0.05, 0.95))
image( x1, x2, trend, zlim = scale.range, main = 'Trend', col = scale.color)
image( x1, x2, y, zlim = scale.range, main = 'Data', col = scale.color)
par(old.par)
# the legend can be added to any plot...
splot(slim = scale.range, col = scale.color, legend.shrink = 0.7, add = TRUE)

## note that argument 'zlim' in 'image' corresponds with 'slim' in 'sxxxx' functions.