Function to add a space between the legend title and the legend values on a ggplot plot

add_legend_title_space.R

#' Add space between the legend title and the legend items
#' @param gplt A plot created with ggplot
#' @param x How much space to add, given in x_unit units (default 0.5 lines)
#' @param x_unit Unit of x, "line" by default
#' @return A gtable object that can be plotted using plot()
#' @examples
#' library(ggplot2)
#' gplt <- ggplot(iris) + geom_point(aes(x=Sepal.Length, y = Sepal.Width, color = Species))
#' plt_gtable <- add_legend_title_space(gplt)
#' plot(plt_gtable)
add_legend_title_space <- function(gplt, x=0.5, x_unit="line") {
  p_table <- ggplot2::ggplot_gtable(ggplot2::ggplot_build(gplt))
  
  # Adapted from
  # https://stackoverflow.com/a/32276573/446149
  ## extract legend
  leg <- which(vapply(p_table$grobs, function(x) x$name, character(1)) == "guide-box")
  # The legend is represented as a table
  legend_table <- p_table$grobs[[leg]]$grobs[[1]]
  # Where is the title?
  leg_title <- which(legend_table$layout$name == "title")
  ## this is the tricky part !
  ## adds a row after the title
  p_table$grobs[[leg]]$grobs[[1]] <-
    gtable::gtable_add_rows(p_table$grobs[[leg]]$grobs[[1]],
                            grid::unit(x, x_unit),
                            pos=p_table$grobs[[leg]]$grobs[[1]]$layout$b[leg_title])
  return(p_table)
}

Example: