Increase space between legend keys without increasing legend keys

Question

This is a follow-up on https://stackoverflow.com/questions/32275113

The problem is to tweak the legend elements to increase the space between legend keys without simultaneously extending the legend keys themselves. The solution is likely to be in tweaking the correct legend theme option.

Desired result: more vertical space between the legend key text labels, but without stretching the legend key lines.

d <- data.frame(x = mtcars$mpg, y = 0.10)
vlines <- rbind(aggregate(d[1], d[2], mean), 
                aggregate(d[1], d[2], median))
vlines$stat <- rep(c("mean", "median"), each = nrow(vlines)/2)
library("ggplot2")
ggplot(data = d, aes(x = x, y = ..density..)) + 
    geom_histogram(fill = "lightblue", color = "black") + 
    geom_vline(data = vlines, mapping = aes(xintercept = x, colour = stat), 
            show.legend = TRUE) +
    theme(legend.direction = "vertical", 
        legend.position = "right",
        #          legend.key = element_rect(size = 2),
        legend.key.size = unit(3, "cm"),
        #          legend.key.width = unit(2, "cm"),
        #          legend.key.height = unit(1, "cm")
        )

Increasing legend.key.size, as suggested in answers to the linked question (see above), has the undesired side effect of increasing the vertical lines as well.

Edit Based on PoGibas's clever workaround, here's a screenshot of the desired result, included here to make sure the purpose is clear:

Following PoGibas, I used: shape = 73, legend.key.height = unit(2, "cm") and size = 6 inside the color guide.

Answer 1

One solution is to replace lines with points (requires additional geom layer):

Create plot with invisible points (size = 0 and rectangle shape shape = 15).

p <- ggplot(d, aes(x, ..density..)) + 
    geom_histogram(fill = "lightblue", color = "black") + 
    geom_vline(data = vlines, mapping = aes(xintercept = x, colour = stat)) +
    geom_point(data = vlines, aes(0, 0, colour = stat), size = 0, shape = 15)

Add legend theme to:

• Mask background color in legend (legend.key = element_rect(fill = "white"))
• Create large legend (legend.key.height = unit(3, "cm"))
• Remove lines (linetype = 0) and make large points (size = 5)

Code:

p + 
    theme(legend.direction = "vertical", 
          legend.position = "right",
          legend.key = element_rect(fill = "white"),
          legend.key.height = unit(3, "cm")) +
    guides(color = guide_legend(override.aes = list(linetype = 0, size = 5)))

PS.:

• This is not a perfect solution as there's a gap between legend label and boxes.
• If you want lines instead of rectangles use shape = 73