Draws the layer twice: first in paper colour with width mm added to each side of every stroke, then as is. Use it
for a line that crosses a dot field, such as a step curve over a screened confidence band or contours over an
elevation field. Keep the width small: 0.09 mm (0.25 pt, the printable minimum) against dots, 0.15 mm against a
line screen. A wider halo looks like a second line.
Arguments
- layer
A ggplot2 layer drawing lines, paths, steps, contours or points; a list holding one; or a whole plot or patchwork, in which case every layer in it is wrapped. The object handed in is left alone.
- width
Halo width in mm on each side of the stroke.
- colour
Halo colour. Use the paper colour of the plot; the default is white.
