Add labels to the center of a geom_curve line (ggplot)

Question

Is there any way to add a label on or near the center of a geom_curve line? Currently, I can only do so by labeling either the start or end point of the curve.

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)

I would love some way to automatically label the curve near coordinates x=1.75, y=1.5. Is there a solution out there I haven't seen yet? My intended graph is quite busy, and labeling the origin points makes it harder to see what's happening, while labeling the arcs would make a cleaner output.

Answer 1

I've come to a solution for this problem. It's large and clunky, but effective.

The core problem is that geom_curve() does not draw a set path, but it moves and scales with the aspect ratio of the plot window. So short of locking the aspect ratio with coord_fixed(ratio=1) there is no way I can easily find to predict where the midpoint of a geom_curve() segment will be.

So instead I set about finding midpoint for a curve, and then forcing the curve to go through that point which I would later label. To find the midpoint I had to copy two functions from the grid package:

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)
}

With that in place, I calculated a midpoint for each record

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)

I then make the graph, but draw two separate curves. One from the origin to the calculated midpoint, and another from the midpoint to the destination. The angle and curvature settings for both finding the midpoint and drawing these curves are tricky to keep the result from obviously looking like two different curves.

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)

Though the answer isn't ideal or elegant, it scales with a large number of records.