Compute dimensions of the largest fixed-aspect ggplot that fits inside the given maximum height and width.

plotshape.r

# The problem: Plotting from R to PNG requires that you specify x and y
# dimensions, which therefore also fixes the aspect ratio of
# the whole image. In most of my plots, I want a fixed *panel* aspect ratio, 
# but the overall dimensions of the full *plot* still depend on the dimensions 
# of other plot elements: axes, legends, titles, etc.
# In a facetted ggplot, this gets even trickier: "OK, three panels, each
# with aspect ratio of 1.5, that adds up to... wait, will every panel
# have its own y-axis, or just the leftmost one?"

# ggplot apparently computes absolute dimensions for everything EXCEPT 
# the panels, so the approach here is to build the plot inside a 
# throwaway device, subtract off the parts used for non-panel elements,
# then divide the remainder up between panels. 
# One dimension will be constrained by the size of the throwaway
# device, and we can then calculate the other dimension from the 
# panel aspect ratio.
	
# Known issues: 
# 1. It's inefficient, because we have to built the plot twice.
#	Since dimensions aren't calculated until the plot is built, 
#	I don't know of any way around this. Do you?
# 2. For me at least, it seems to create a new, empty Rplots.pdf on exit.
#	Probably related to stackoverflow.com/questions/17348359?

# Required packages: ggplot for all of ggplot, grid for convertHeight and convertWidth.

get_dims = function(ggobj, maxheight, maxwidth=maxheight, units="in", ...){
	####
	# Computes dimensions of the largest fixed-aspect ggplot that 
	# fits inside the given maximum height and width.
	#
	# Arguments:
	# ggobj
	#	A ggplot or gtable object.
	# maxheight, maxwidth 
	#	Numeric, giving argest allowable dimensions of the plot. 
	#	The final image will exactly match one of these and not exceed 
	#	the other.
	# units
	#	Character, giving units for the dimensions. 
	#	Must be recognized by BOTH png() and grid::convertUnit,
	#	probably limited to "in", "cm", "mm".
	#	Note especially that "px" does NOT work.
	# ...
	#	Other arguments passed to png() when setting up the throwaway plot.
	#
	# Intended usage: 
	#	a=ggplot(...)
	#	d=get_dims(a, maxheight=1200, maxwidth=800)
	#	png("plot_of_a.png", height=d$height, width=d$width)
	#	plot(a)
	#	dev.off()
 	####

	 # open a plotting device to do the calculations inside
	tmpf = tempfile(pattern="dispos-a-plot", fileext= ".png")
	png(filename=tmpf, height=maxheight, width=maxwidth, units=units, ...)
	on.exit({dev.off(); unlink(tmpf)})

	# Doesn't give us the real aspect ratio, but if both are unset
	# then we can exit without building -- the plot will fill the whole area.
	if ("ggplot" %in% class(ggobj)
			&& is.null(ggobj$theme$aspect.ratio)
			&& is.null(ggobj$coordinates$ratio)){
		return(c(height=maxheight, width=maxwidth))}

	if("ggplot" %in% class(ggobj)){
		g = ggplot_gtable(ggplot_build(g))
	}else if ("gtable" %in% class(ggobj)){
		g = ggobj
	}else{
		stop(paste(
			"Don't know how to get sizes for object of class",
			class(ggobj)))
	}

	panel_layout = g$layout[grepl("panel", g$layout$name),]
	n_rows = length(unique(panel_layout$t))
	n_cols = length(unique(panel_layout$l))

	# panels have unit "null", but they carry ratio information anyway.
	gw_units <- sapply(g$widths, attr, "unit")
  	gh_units <- sapply(g$heights, attr, "unit")

	asp = (unlist(g$heights[gh_units == "null"])
		/ unlist(g$widths[gw_units == "null"]))
	if(length(unique(asp)) > 1){
		stop("panels have different aspect ratios?!")}
	asp = asp[1]

	# convertUnit treats null units as 0, 
	# so this sum gives the dimensions of *non-panel* grobs.
	known_hts = sum(grid::convertHeight(g$heights, units, valueOnly=T))
	known_wds = sum(grid::convertWidth(g$widths, units, valueOnly=T))
	free_ht = maxheight - known_hts
	free_wd = maxwidth - known_wds

	# Whew. Now we're ready to calculate image dimensions.
	height = maxheight
	width = ((free_ht / n_rows) / asp * n_cols) + known_wds
	if (width > maxwidth){
		width = maxwidth
		height = ((free_wd / n_cols) * asp * n_rows) + known_hts}

	return(list(height=height, width=width))
}


gg_png = function(
		ggobj,
		filename,
		maxheight=400,
		maxwidth=400,
		res=300,
		units="in",
		...){
	# Plot a ggplot or gtable object to a correctly-shaped PNG.
	# This is a thin wrapper around get_dims for the simplest use case. 
	# For more complex setups, use get_dims to compute the device size 
	# and then handle the plotting yourself.
	dims = get_dims(
		ggobj=ggobj,
		maxheight=maxheight,
		maxwidth=maxwidth,
		res=res,
		units=units,
		...)
	png(
		filename=filename,
		height=dims$height,
		width=dims$width,
		res=res,
		units=units, ...)
	plot(ggobj)
	dev.off()
}