Paired Pre–Post Spaghetti Plot — plot

Creates a publication-ready paired (within-subject) plot for pre–post designs. Individual participant trajectories are shown as connected lines with optional mean overlay. Designed for thermometer-style scales or other continuous outcomes.

Usage

plot_prepost(
  data,
  pre,
  post,
  group = NULL,
  colors = NULL,
  pre_label = "Pre",
  post_label = "Post",
  y_label = NULL,
  title = NULL,
  point_size = 2,
  line_alpha = 0.35,
  point_alpha = 0.5,
  jitter = 0,
  show_mean = FALSE
)

Arguments

data: A data frame in wide format containing the pre and post variables.
pre: Unquoted name of the pre-intervention variable.
post: Unquoted name of the post-intervention variable.
group: Optional character string specifying a grouping variable. When provided, individual trajectories and (if requested) mean lines are colored by group.
colors: Optional named character vector of colors for the group variable. Names must match group levels.
pre_label: Character. Label displayed for the pre condition.
post_label: Character. Label displayed for the post condition.
y_label: Character. Label for the y-axis. Defaults to NULL.
title: Character. Optional plot title.
point_size: Numeric. Size of individual points. Default is 2.
line_alpha: Numeric between 0 and 1. Transparency of paired lines.
point_alpha: Numeric between 0 and 1. Transparency level for individual points. Lower values are recommended for large samples to reduce overplotting.
jitter: Numeric. Horizontal jitter width applied to individual points (no vertical jitter). Useful for large samples to improve visibility of overlap.
show_mean: Logical. If TRUE, overlays the mean trajectory.

Value

A ggplot object.

Details

The function reshapes wide data internally using tidy evaluation. It is intended for within-subject comparisons (e.g., pre–post RCT outcomes). No axis limits are imposed; scaling adapts to the data range.

Examples

if (requireNamespace("dplyr", quietly = TRUE) &&
    requireNamespace("tidyr", quietly = TRUE) &&
    requireNamespace("ggplot2", quietly = TRUE)) {

  df <- data.frame(
    pre = rnorm(50, 25, 5),
    post = rnorm(50, 15, 5)
  )

  plot_prepost(
    df,
    "pre",
    "post",
    pre_label = "Before",
    post_label = "After",
    title = "Reduction in Affective Polarization",
    show_mean = TRUE
  )
}