ggplot2中的中心对齐图例标题和图例键，用于长图例标题

Question

当图例标题很长时，我很难将图例中心的标题相对于图例键对齐。 a year ago有一个问题适用于短标题，但它似乎不适用于长标题。

示例，首先使用简短的图例标题：

library(ggplot2)
ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) + geom_point(size = 3) +
  scale_color_distiller(palette = "YlGn", type = "seq", direction = -1,
                        name = "A") +
  theme(legend.title.align = 0.5)

所有内容都符合预期，图例标题位于图例键的上方。

现在与长篇传奇标题相同：

ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) + geom_point(size = 3) +
  scale_color_distiller(palette = "YlGn", type = "seq", direction = -1,
                        name = "Long legend heading\nShould be centered") +
  theme(legend.title.align = 0.5)

我们可以看到文本中心与自身对齐，但与图例键无关。我尝试修改其他主题选项，例如legend.justification = "center"，但似乎没有人从图例框中最左侧的位置移动键。

几条评论：

• 我几天前正在运行ggplot2，v2.2.1.9000的开发版本。

• 我特别需要一个连续色阶调色板的解决方案。

Answer 1

我破解了类似于baptiste在上述评论之一中描述的方法的源代码：将颜色条/标签/刻度grobs放入子gtable中，并将其定位为具有相同的行跨度/列跨度（取决于在图例的方向上）作为标题。

它仍然是一个hack，但我想将其视为“整个会话一次hack”的方法，而不必为每个情节手动重复这些步骤。

具有不同标题宽度/标题位置/图例方向的演示

plot.demo <- function(title.width = 20,
                      title.position = "top",
                      legend.direction = "vertical"){
  ggplot(iris, 
         aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) + 
    geom_point(size = 3) +
    scale_color_distiller(palette = "YlGn",
                          name = stringr::str_wrap("Long legend heading should be centered",
                                                   width = title.width), 
                          guide = guide_colourbar(title.position = title.position),
                          direction = -1) +
    theme(legend.title.align = 0.5,
          legend.direction = legend.direction)
}

cowplot::plot_grid(plot.demo(),
                   plot.demo(title.position = "left"),
                   plot.demo(title.position = "bottom"),
                   plot.demo(title.width = 10, title.position = "right"),
                   plot.demo(title.width = 50, legend.direction = "horizontal"),
                   plot.demo(title.width = 10, legend.direction = "horizontal"),
                   ncol = 2)

这也适用于多个颜色条图例：

ggplot(iris, 
       aes(x=Sepal.Length, y=Sepal.Width, 
           color=Petal.Width, fill = Petal.Width)) + 
  geom_point(size = 3, shape = 21) +
  scale_color_distiller(palette = "YlGn",
                        name = stringr::str_wrap("Long legend heading should be centered",
                                                 width = 20),
                        guide = guide_colourbar(title.position = "top"),
                        direction = -1) +
  scale_fill_distiller(palette = "RdYlBu",
                       name = stringr::str_wrap("A different heading of different length",
                                                width = 40),
                       direction = 1) +
  theme(legend.title.align = 0.5,
        legend.direction = "vertical",
        legend.box.just = "center")

（旁注：legend.box.just = "center"是正确对齐两个图例的原因。我担心了一段时间，因为目前仅列出了“顶部”，“底部”，“左侧”和“右侧”参数值，但事实证明基础grid::valid.just也接受“中心” /“中心”两个参数，但不确定?theme帮助文件中为什么未明确提及；尽管如此，它确实有效。）

要更改源代码，请运行：

trace(ggplot2:::guide_gengrob.colorbar, edit = TRUE)

并从此更改代码的最后一部分：

  gt <- gtable(widths = unit(widths, "cm"), heights = unit(heights, 
    "cm"))
  ... # omitted
  gt
}

对此：

  # create legend gtable & add background / legend title grobs as before (this part is unchanged)
  gt <- gtable(widths = unit(widths, "cm"), heights = unit(heights, "cm"))
  gt <- gtable_add_grob(gt, grob.background, name = "background", 
                        clip = "off", t = 1, r = -1, b = -1, l = 1)
  gt <- gtable_add_grob(gt, justify_grobs(grob.title, hjust = title.hjust, 
                                          vjust = title.vjust, int_angle = title.theme$angle, 
                                          debug = title.theme$debug), name = "title", clip = "off", 
                        t = 1 + min(vps$title.row), r = 1 + max(vps$title.col), 
                        b = 1 + max(vps$title.row), l = 1 + min(vps$title.col))

  # create child gtable, using the same widths / heights as the original legend gtable
  gt2 <- gtable(widths = unit(widths[1 + seq.int(min(range(vps$bar.col, vps$label.col)), 
                                                 max(range(vps$bar.col, vps$label.col)))], "cm"),
                heights = unit(heights[1 + seq.int(min(range(vps$bar.row, vps$label.row)), 
                                                   max(range(vps$bar.row, vps$label.row)))], "cm"))

  # shift cell positions to start from 1
  vps2 <- vps[c("bar.row", "bar.col", "label.row", "label.col")]
  vps2[c("bar.row", "label.row")] <- lapply(vps2[c("bar.row", "label.row")],
                                            function(x) x - min(unlist(vps2[c("bar.row", "label.row")])) + 1)
  vps2[c("bar.col", "label.col")] <- lapply(vps2[c("bar.col", "label.col")],
                                            function(x) x - min(unlist(vps2[c("bar.col", "label.col")])) + 1)

  # add bar / ticks / labels grobs to child gtable
  gt2 <- gtable_add_grob(gt2, grob.bar, name = "bar", clip = "off",
                         t = min(vps2$bar.row), r = max(vps2$bar.col),
                         b = max(vps2$bar.row), l = min(vps2$bar.col))
  gt2 <- gtable_add_grob(gt2, grob.ticks, name = "ticks", clip = "off",
                         t = min(vps2$bar.row), r = max(vps2$bar.col),
                         b = max(vps2$bar.row), l = min(vps2$bar.col))
  gt2 <- gtable_add_grob(gt2, grob.label, name = "label", clip = "off",
                         t = min(vps2$label.row), r = max(vps2$label.col),
                         b = max(vps2$label.row), l = min(vps2$label.col))

  # add child gtable back to original legend gtable, taking tlrb reference from the
  # rowspan / colspan of the title grob if title grob spans multiple rows / columns.
  gt <- gtable_add_grob(gt, justify_grobs(gt2, hjust = title.hjust, 
                                          vjust = title.vjust), 
                        name = "bar.ticks.label", clip = "off", 
                        t = 1 + ifelse(length(vps$title.row) == 1, 
                                       min(vps$bar.row, vps$label.row),
                                       min(vps$title.row)), 
                        b = 1 + ifelse(length(vps$title.row) == 1, 
                                       max(vps$bar.row, vps$label.row),
                                       max(vps$title.row)), 
                        r = 1 + ifelse(length(vps$title.col) == 1, 
                                       min(vps$bar.col, vps$label.col),
                                       max(vps$title.col)), 
                        l = 1 + ifelse(length(vps$title.col) == 1, 
                                       max(vps$bar.col, vps$label.col),
                                       min(vps$title.col)))
  gt
}

要撤消更改，请运行：

untrace(ggplot2:::guide_gengrob.colorbar)

使用的软件包版本：ggplot2 3.2.1

Answer 2

我找到了解决方案。它需要对grob树进行一些挖掘，如果有多个图例，它可能无法工作，但除非出现更好的结果，否则这似乎是一个合理的解决方案。

library(ggplot2)
library(gtable)
library(grid)

p <- ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color=Petal.Width)) + 
  geom_point(size = 3) +
  scale_color_distiller(palette = "YlGn", type = "seq", direction = -1,
                        name = "Long legend heading\nShould be centered") +
  theme(legend.title.align = 0.5)

# extract legend
g <- ggplotGrob(p)
grobs <- g$grobs
legend_index <- which(sapply(grobs, function(x) x$name) == "guide-box")
legend <- grobs[[legend_index]]

# extract guides table
guides_index <- which(sapply(legend$grobs, function(x) x$name) == "layout")
guides <- legend$grobs[[guides_index]]

# add extra column for spacing
# guides$width[5] is the extra spacing from the end of the legend text
# to the end of the legend title. If we instead distribute it 50:50 on
# both sides, we get a centered legend
guides <- gtable_add_cols(guides, 0.5*guides$width[5], 1)
guides$widths[6] <- guides$widths[2]
title_index <- guides$layout$name == "title"
guides$layout$l[title_index] <- 2

# reconstruct legend and write back
legend$grobs[[guides_index]] <- guides
g$grobs[[legend_index]] <- legend

grid.newpage()
grid.draw(g)

Answer 3

您必须更改源代码。目前在视口（gtable）中为computes the widths for the title grob and the bar+labels和left-justifies the bar+labels。这是硬编码的。