定义 /修改draw_key的数据

Question

这个问题提出了回答如何在ggplot2传奇中向后和向前方向显示箭头？

我认为，我认为自动定义箭头方向的好方法是传递此值，在这里，“方向” ，作为美学，并相应地交换segmentsGrob的方向。看来draw_key _...使用像stat和Geom一样的“数据”和“ params”，但它只是一个带有单个行的数据框架，仅包含四个变量。

## browser()
# Browse[1]> data
# colour size linetype alpha
# 1 #F8766D  0.5        1    NA

我还尝试过 - 没有成功 - 将方向添加为美学：

GeomArrow <- ggproto(NULL, GeomSegment)
GeomArrow$required_aes <- c("x", "y", "xend", "yend", "direction")

如何修改draw_key中使用的数据？

library(ggplot2)
foo <- structure(list(direction = c("backward", "forward"), 
                      x = c(0, 0), xend = c(1, 1), y = c(1, 1.2), 
                      yend = c(1, 1.2)), row.names = 1:2, class = "data.frame")

StatArrow <- ggproto(NULL, StatIdentity)
StatArrow$compute_layer <- function (self, data, params, layout) 
{
    swap <- data$direction == "backward"
    v1 <- data$x
    v2 <- data$xend
    data$x[swap] <- v2[swap]
    data$xend[swap] <- v1[swap]
    data
}

# draw_key_segment_custom <- function(data, params = list(direction), size) {
## ... 
## the idea was to simply switch x0 and x1 in the segmentsGrob depending on the new aesthetic
## This does not work because "data$direction" does not exist
## something like 
  # if(data$direction == "backward") {
  #   x0 = 0.9
  #   x1 = 0.1
  # } else {
  #   x0 = 0.1
  #   x1 = 0.9
  # }
  ## then 
  # grid::segmentsGrob(x0, 0.5, x1, 0.5,
                     ##...)
## but 
## browser()
# Browse[1]> data
# colour size linetype alpha
# 1 #F8766D  0.5        1    NA

# }

## direction as an aesthetic works despite this warning
ggplot() +
  geom_segment(data = foo,
               stat = "arrow",
               aes(x, y, xend = xend, yend = yend, col = direction, 
                   direction = direction),
               arrow = arrow(length = unit(0.3, "cm"), type = "closed")
             ## does not work, as per above
             # key_glyph = "segment_custom"
)
#> Warning: Ignoring unknown aesthetics: direction

^{由 reprex软件包（v2.0.1）}

Answer 1

我认为您首先认为是正确的，那将是GEOM的quiend_aes。如果我们制作了这样的怪异：

library(ggplot2)
library(rlang)

foo <- structure(list(direction = c("backward", "forward"), 
                      x = c(0, 0), xend = c(1, 1), y = c(1, 1.2), 
                      yend = c(1, 1.2)), row.names = 1:2, class = "data.frame")

GeomArrow <- ggproto(
  NULL, GeomSegment,
  required_aes = c("x", "y", "xend", "yend", "direction"),
  draw_layer = function (self, data, params, layout, coord) 
  {
    swap <- data$direction == "backward"
    v1 <- data$x
    v2 <- data$xend
    data$x[swap] <- v2[swap]
    data$xend[swap] <- v1[swap]
    GeomSegment$draw_layer(data, params, layout, coord)
  }
)

并声明适当的密钥绘图功能

draw_key_arrow = function(data, params, size) {
  grid::segmentsGrob(
    x0 = ifelse(data$direction == "forward", 0.1, 0.9),
    x1 = ifelse(data$direction == "forward", 0.9, 0.1),
    # Rest is just like vanilla
    y0 = 0.5, y1 = 0.5,
    gp = grid::gpar(
      col  = alpha(data$colour %||% data$fill %||% "black", data$alpha),
      fill = alpha(params$arrow.fill %||% data$colour %||% data$fill %||% 
                     "black", data$alpha),
      lwd = (data$size %||% 0.5) * .pt,
      lty = data$linetype %||% 1, lineend = "butt"
    ),
    arrow = params$arrow
  )
}

，那么我们应该能够渲染图如果（!!!）我们还将Direction审美映射到带有传奇的比例。

ggplot() +
  stat_identity(
    geom = GeomArrow,
    data = foo,
    aes(x, y, xend = xend, yend = yend, col = direction, direction = direction),
    arrow = arrow(length = unit(0.3, "cm"), type = "closed"),
    key_glyph = draw_key_arrow
  ) +
  # Hijacking a generic identity scale by specifying aesthetics
  scale_colour_identity(aesthetics = "direction", guide = "legend")

^{由 reprex软件包（v2.0.1）}

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