将标签添加到geom_curve行的中心（ggplot）

Question

有没有办法在geom_curve线的中心或附近添加标签？目前，我只能通过标记曲线的起点或终点来实现。

library(tidyverse)
library(ggrepel)

df <- data.frame(x1 = 1, y1 = 1, x2 = 2, y2 = 3, details = "Object Name")

ggplot(df, aes(x = x1, y = y1, label = details)) +
  geom_point(size = 4) +
  geom_point(aes(x = x2, y = y2),
             pch = 17, size = 4) +
  geom_curve(aes(x = x1, y = y1, xend = x2, yend = y2)) +
  geom_label(nudge_y = 0.05) +
  geom_label_repel(box.padding = 2)

我希望能够在坐标x = 1.75，y = 1.5附近自动标记曲线。那里有一个解决方案我还没见过吗？我想要的图表非常繁忙，标记原点会使得查看正在发生的事情变得更加困难，而标记弧线会使输出更清晰。

Answer 1

我已经找到了解决这个问题的方法。它庞大而笨重但有效。

核心问题是geom_curve()没有绘制设定路径，但它会随着绘图窗口的纵横比移动和缩放。如果没有将宽高比与coord_fixed(ratio=1)锁定，我就无法轻易找到预测geom_curve()段的中点的位置。

所以我开始寻找曲线的中点，然后强制曲线通过我后来标记的那个点。要找到中点，我必须从grid package复制两个函数：

library(grid)
library(tidyverse)
library(ggrepel)

# Find origin of rotation
# Rotate around that origin
calcControlPoints <- function(x1, y1, x2, y2, curvature, angle, ncp,
                              debug=FALSE) {
  # Negative curvature means curve to the left
  # Positive curvature means curve to the right
  # Special case curvature = 0 (straight line) has been handled
  xm <- (x1 + x2)/2
  ym <- (y1 + y2)/2
  dx <- x2 - x1
  dy <- y2 - y1
  slope <- dy/dx

  # Calculate "corner" of region to produce control points in
  # (depends on 'angle', which MUST lie between 0 and 180)
  # Find by rotating start point by angle around mid point
  if (is.null(angle)) {
    # Calculate angle automatically
    angle <- ifelse(slope < 0,
                    2*atan(abs(slope)),
                    2*atan(1/slope))
  } else {
    angle <- angle/180*pi
  }
  sina <- sin(angle)
  cosa <- cos(angle)
  # FIXME:  special case of vertical or horizontal line ?
  cornerx <- xm + (x1 - xm)*cosa - (y1 - ym)*sina
  cornery <- ym + (y1 - ym)*cosa + (x1 - xm)*sina

  # Debugging
  if (debug) {
    grid.points(cornerx, cornery, default.units="inches",
                pch=16, size=unit(3, "mm"),
                gp=gpar(col="grey"))
  }

  # Calculate angle to rotate region by to align it with x/y axes
  beta <- -atan((cornery - y1)/(cornerx - x1))
  sinb <- sin(beta)
  cosb <- cos(beta)
  # Rotate end point about start point to align region with x/y axes
  newx2 <- x1 + dx*cosb - dy*sinb
  newy2 <- y1 + dy*cosb + dx*sinb

  # Calculate x-scale factor to make region "square"
  # FIXME:  special case of vertical or horizontal line ?
  scalex <- (newy2 - y1)/(newx2 - x1)
  # Scale end points to make region "square"
  newx1 <- x1*scalex
  newx2 <- newx2*scalex

  # Calculate the origin in the "square" region
  # (for rotating start point to produce control points)
  # (depends on 'curvature')
  # 'origin' calculated from 'curvature'
  ratio <- 2*(sin(atan(curvature))^2)
  origin <- curvature - curvature/ratio
  # 'hand' also calculated from 'curvature'
  if (curvature > 0)
    hand <- "right"
  else
    hand <- "left"
  oxy <- calcOrigin(newx1, y1, newx2, newy2, origin, hand)
  ox <- oxy$x
  oy <- oxy$y

  # Calculate control points
  # Direction of rotation depends on 'hand'
  dir <- switch(hand,
                left=-1,
                right=1)
  # Angle of rotation depends on location of origin
  maxtheta <- pi + sign(origin*dir)*2*atan(abs(origin))
  theta <- seq(0, dir*maxtheta,
               dir*maxtheta/(ncp + 1))[c(-1, -(ncp + 2))]
  costheta <- cos(theta)
  sintheta <- sin(theta)
  # May have BOTH multiple end points AND multiple
  # control points to generate (per set of end points)
  # Generate consecutive sets of control points by performing
  # matrix multiplication
  cpx <- ox + ((newx1 - ox) %*% t(costheta)) -
    ((y1 - oy) %*% t(sintheta))
  cpy <- oy + ((y1 - oy) %*% t(costheta)) +
    ((newx1 - ox) %*% t(sintheta))

  # Reverse transformations (scaling and rotation) to
  # produce control points in the original space
  cpx <- cpx/scalex
  sinnb <- sin(-beta)
  cosnb <- cos(-beta)
  finalcpx <- x1 + (cpx - x1)*cosnb - (cpy - y1)*sinnb
  finalcpy <- y1 + (cpy - y1)*cosnb + (cpx - x1)*sinnb

  # Debugging
  if (debug) {
    ox <- ox/scalex
    fox <- x1 + (ox - x1)*cosnb - (oy - y1)*sinnb
    foy <- y1 + (oy - y1)*cosnb + (ox - x1)*sinnb
    grid.points(fox, foy, default.units="inches",
                pch=16, size=unit(1, "mm"),
                gp=gpar(col="grey"))
    grid.circle(fox, foy, sqrt((ox - x1)^2 + (oy - y1)^2),
                default.units="inches",
                gp=gpar(col="grey"))
  }

  list(x=as.numeric(t(finalcpx)), y=as.numeric(t(finalcpy)))
}

calcOrigin <- function(x1, y1, x2, y2, origin, hand) {
  # Positive origin means origin to the "right"
  # Negative origin means origin to the "left"
  xm <- (x1 + x2)/2
  ym <- (y1 + y2)/2
  dx <- x2 - x1
  dy <- y2 - y1
  slope <- dy/dx
  oslope <- -1/slope
  # The origin is a point somewhere along the line between
  # the end points, rotated by 90 (or -90) degrees
  # Two special cases:
  # If slope is non-finite then the end points lie on a vertical line, so
  # the origin lies along a horizontal line (oslope = 0)
  # If oslope is non-finite then the end points lie on a horizontal line,
  # so the origin lies along a vertical line (oslope = Inf)
  tmpox <- ifelse(!is.finite(slope),
                  xm,
                  ifelse(!is.finite(oslope),
                         xm + origin*(x2 - x1)/2,
                         xm + origin*(x2 - x1)/2))
  tmpoy <- ifelse(!is.finite(slope),
                  ym + origin*(y2 - y1)/2,
                  ifelse(!is.finite(oslope),
                         ym,
                         ym + origin*(y2 - y1)/2))
  # ALWAYS rotate by -90 about midpoint between end points
  # Actually no need for "hand" because "origin" also
  # encodes direction
  # sintheta <- switch(hand, left=-1, right=1)
  sintheta <- -1
  ox <- xm - (tmpoy - ym)*sintheta
  oy <- ym + (tmpox - xm)*sintheta

  list(x=ox, y=oy)
}

有了这个，我计算了每个记录的中点

df <- data.frame(x1 = 1, y1 = 1, x2 = 10, y2 = 10, details = "Object Name")

df_mid <- df %>% 
  mutate(midx = calcControlPoints(x1, y1, x2, y2, 
                                  angle = 130, 
                                  curvature = 0.5, 
                                  ncp = 1)$x) %>% 
  mutate(midy = calcControlPoints(x1, y1, x2, y2, 
                                  angle = 130, 
                                  curvature = 0.5, 
                                  ncp = 1)$y)

然后我制作图表，但绘制两条单独的曲线。一个从原点到计算中点，另一个从中点到目的地。找到中点和绘制这些曲线的角度和曲率设置很难保持结果显然看起来像两条不同的曲线。

ggplot(df_mid, aes(x = x1, y = y1)) +
  geom_point(size = 4) +
  geom_point(aes(x = x2, y = y2),
             pch = 17, size = 4) +
  geom_curve(aes(x = x1, y = y1, xend = midx, yend = midy),
             curvature = 0.25, angle = 135) +
  geom_curve(aes(x = midx, y = midy, xend = x2, yend = y2),
             curvature = 0.25, angle = 45) +
  geom_label_repel(aes(x = midx, y = midy, label = details),
                   box.padding = 4,
                   nudge_x = 0.5,
                   nudge_y = -2)

虽然答案不理想或优雅，但它会随着大量记录而扩展。

Answer 2

也许注释会有所帮助（参见：http://ggplot2.tidyverse.org/reference/annotate.html）

library(tidyverse)
library(ggrepel)

df <- data.frame(x1 = 1, y1 = 1, x2 = 2, y2 = 3, details = "Object Name")

ggplot(df, aes(x = x1, y = y1, label = details)) +
  geom_point(size = 4) +
  geom_point(aes(x = x2, y = y2),
             pch = 17, size = 4) +
  geom_curve(aes(x = x1, y = y1, xend = x2, yend = y2)) +
  geom_label(nudge_y = 0.05) +
  geom_label_repel(box.padding = 2) +
  annotate("label", x=1.75, y=1.5, label=df$details)