Question

The gridExtra package adds a grob of class “pattern” that lets one fill rectangles with patterns. For example,

library(gridExtra)
grid.pattern(pattern = 1)

creates a box filled with diagonal lines. I want to create a stack of panels in which each panel is filled with these diagonal lines. This is easy:

library(lattice); library(gridExtra)
examplePlot <- xyplot(
  1 ~ 1 | 1:2, 
  panel = function () grid.pattern(pattern = 1),
  layout = c(1, 2),

  # Remove distracting visual detail
  scales = list(x=list(draw=FALSE), y=list(draw=FALSE)),
  strip = FALSE, xlab = '', ylab = ''
)
print(examplePlot)

The problem is that the diagonal lines aren’t aligned across panels. That is, there is a visual “break” where the bottom of the first panel meets the top of the second panel: at that point, the lines don’t line up. This is the problem that I want to fix.

I can eliminate most of the visual break by adding the argument pattern.offset = c(.2005, 0) to the grid.pattern call, and making sure that it applies only to the bottom panel. But this solution doesn’t generalize. For example, if I change the pattern (e.g., by using the granularity argument to grid.pattern), this solution won’t work. Is there a more general fix?

Answer 1

To make this work, you’ll have to take charge of setting the panel.height argument used by print.trellis. (To see why, try resizing your plotting device after running your example code: as the size of the device and the panels changes, so does the matching/mismatching of the lines):

## Calculate vertical distance (in mm) between 45 degree diagonal lines
## spaced 5mm apart (the default distance for grid.pattern).
vdist <- 5 * sqrt(2)

nLines <- 8L    ## can be any integer
panelHeight <- list(x = nLines*vdist, units = "mm", data = NULL)

## Plot it
print(examplePlot, panel.height=panelHeight)