ggplot2：图外注解的边距

Question

I've got almost the exact same problem as this question: Multi-row x-axis labels in ggplot line chart . The accepted answer works for me most of the way, but depending on the size of my plot, the annotations sometimes fall off my plot.

I've tried messing around with plot and legend margins relative to the y scale, but it hasn't worked as I'd hoped. This is what I've got so far:

library(magrittr)
library(ggplot2)

test <- structure(list(
    area = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L,
                       2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L),
                     .Label = c("A", "B", "C"),
                     class = "factor"),
    Year = c(2015L, 2015L, 2015L, 2015L, 2016L, 2016L, 2016L, 2016L, 2017L,
             2017L, 2017L, 2015L, 2015L, 2015L, 2015L, 2016L, 2016L, 2016L,
             2016L, 2017L, 2017L, 2017L),
    Quarter = c(1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 1L, 2L, 3L, 4L, 1L,
                2L, 3L, 4L, 1L, 2L, 3L),
    rate = c(0.52, 0.35, 0.23, 0.36, 0.21, 0.17, 0.33, 0.26, 0.3, 0.31, 0.24,
             0.24, 0.29, 0.42, 0.15, 0.36, 0.33, 0.41, 0.27, 0.34, 0.33, 0.25)),
    class = c("tbl_df", "tbl", "data.frame"),
    row.names = c(NA, -22L),
    .Names = c("area", "Year", "Quarter", "rate"))

yMax <- max(test$rate) * 1.1
yMin <- yMax/7

fig2 <- (ggplot(test,
                mapping = aes(x = interaction(Year, Quarter, lex.order = TRUE),
                              y = rate, group = area, color = area)) +
             geom_line() +
             coord_cartesian(ylim = c(0, yMax), expand = TRUE) +
             scale_x_discrete(labels = paste0("Q", rep(1:4, 3))) +
             scale_y_continuous(name = "Rate", expand = c(0,0)) +
             annotate(geom = "text", label = unique(test$Year),
                      x = 2.5 + 4 * (0:2),
                      y = -yMin,
                      size = 5, vjust = 0) +
             annotate(geom = "segment",
                      x = c(4.5, 8.5), xend = c(4.5, 8.5),
                      y = 0, yend = -yMin * 1.1) +
             theme(plot.margin = margin(b = yMin/2, unit = "native"),
                   axis.title.x = element_blank(),
                   panel.background = element_blank(),
                   axis.line = element_line(),
                   legend.position = "bottom",
                   legend.margin = margin(t = 30, unit = "native"),
                   legend.background = element_rect(fill = alpha("red", 0.5)))
) %>%
    ggplotGrob() %>% 
    (function(x) {
        x$layout$clip[x$layout$name == "panel"] <- "off"
        return(x)
    }); grid::grid.draw(fig2)

At one size, the plot looks fine, but if I increase the height of the plot, the years start to clash with the legend, and if I make it even taller, they just disappear off the bottom.

I'm confused as to why the legend margin doesn't scale along with the size of the plot, because if it did I think that would fix my issue. Also, I'm very confused why a legend margin of 30 native units is about the same size as the plot margin of ~0.04 native units?

Plot laid out as I would like (red legend background was just so I could see what was going on):

Taller plot starting to have problems:

Answer 1

@Oliver changing the plot.margin units to lines , in , cm works for me, the issue was with the scaling in native units. Would this solution work for you?

fig2 <- (ggplot(test,
                mapping = aes(x = interaction(Year, Quarter, lex.order = TRUE),
                              y = rate, group = area, color = area)) +
           geom_line() +
           coord_cartesian(ylim = c(0, yMax), expand = TRUE) +
           scale_x_discrete(labels = paste0("Q", rep(1:4, 3))) +
           scale_y_continuous(name = "Rate", expand = c(0,0)) +
           annotate(geom = "text", label = unique(test$Year),
                    x = 2.5 + 4 * (0:2),
                    y = -yMin,
                    size = 5, vjust = 0) +
           annotate(geom = "segment",
                    x = c(4.5, 8.5), xend = c(4.5, 8.5),
                    y = 0, yend = -yMin * 1.1) +
           theme(plot.margin = margin(c(1, 1, 3, 1), unit="lines"),
                 axis.title.x = element_blank(),
                 panel.background = element_blank(),
                 axis.line = element_line(),
                 legend.position = "bottom",
                 legend.margin = margin(c(3, 1, 1, 1), unit="lines"),
                 legend.background = element_rect(fill = alpha(0.5)))
                 ) %>%
  ggplotGrob() %>% 
  (function(x) {
    x$layout$clip[x$layout$name == "panel"] <- "off"
    return(x)
  }); grid.draw(fig2)

I changed the legend.background = element_rect(fill = alpha("red", 0.5)) to legend.background = element_rect(fill = alpha(0.5)) as the upper margin is being shaded red and overlapping with the secondary axis titles.