Draw a border tightly around a composite legend's rendered content

legend_canvas positions its sections from nominal geometry, but text labels overflow their anchor points by an amount that depends on the font, the label strings, and the final output size - none of which are known when the layers are built. A border drawn from that nominal geometry therefore clips the labels. legend_box() sidesteps this by rendering the legend once at the intended output size, measuring the true pixel extent of the drawn content, mapping it back to data coordinates, and adding a border rectangle around it.

Because it renders once to measure, the border reflects exactly what will be drawn - long labels, mixed fonts, any content - with no per-figure hand tuning.

Usage

legend_box(
  plot,
  width,
  height,
  padding = c(0.02, 0.12),
  colour = "black",
  linewidth = 0.7,
  fill = NA,
  threshold = 150,
  dpi = 150
)

Arguments

plot

A ggplot (typically legend_canvas output) whose coordinate system supplies fixed xlim/ylim (legend_canvas() always does). Add legend_box() last, after any other layers, so it encloses everything.

width, height

Physical size in inches of the region the legend will be drawn in for the final export - pass the same values used there so the measured text width matches. For a legend_strip legend this is the full figure width and the strip height.

padding

Length-2 numeric: gap between content and border, as fractions of the measured content width and height (default c(0.02, 0.12)).

colour

Border colour (default "black").

linewidth

Border line width (default 0.7).

fill

Border fill (default NA, i.e. transparent).

threshold

Colour-channel cutoff (0-255) for detecting content; a pixel counts as content when its darkest red, green or blue channel is below this value (default 150). Testing the darkest channel, not the gray level, keeps light saturated colours such as yellow inside the border. Very pale fills (all channels above the cutoff) are not detected.

dpi

Resolution of the internal measurement render (default 150); higher is more precise but slower.

Value

plot with a border ggplot2::annotate("rect", ...) layer added.

See also

legend_canvas, legend_strip