add custom x axis with lines to connect the labels

gistfile1.txt

library(ggplot2)
library(patchwork)

add_custom_xaxis <- function(plot, size_ratio = 6) {
  plot_build <- ggplot2::ggplot_build(plot)
  x_scale_type <- class(plot_build$layout$panel_scales_x[[1]])[1]
  x_data <- plot_build$data[[1]]$x
  x_label <- p$labels$x
  
  # Handle different scale types
  if (grepl("discrete", x_scale_type)) {
    # For discrete scales
    order_levels <- levels(factor(x_data))
    if (is.null(order_levels)) {
      order_levels <- unique(as.character(x_data))
    }
  } else {
    x_data_rounded <- round(x_data, 2)  # Round to 2 decimal places
    order_levels <- unique(as.character(x_data_rounded))
    order_levels <- order_levels[order(as.numeric(order_levels))]
  }
  order_levels <- factor(order_levels, levels = order_levels)
  rev_order_levels <- factor(order_levels, levels = rev(order_levels))
  help_table <- data.frame(
    order_levels = order_levels,
    rev_order_levels = rev_order_levels,
    stringsAsFactors = TRUE
  )
  leg <- ggplot2::ggplot(help_table, ggplot2::aes(x = order_levels, y = rev_order_levels)) + 
    ggplot2::geom_segment(ggplot2::aes(xend = 0.5, 
                                       yend = rev_order_levels)) + 
    ggplot2::geom_segment(ggplot2::aes(xend = as.numeric(order_levels), 
                                       yend = max(as.numeric(rev_order_levels)) + 0.5)) + 
    ggplot2::xlab(x_label) + 
    ggplot2::theme_void() + 
    ggplot2::theme(axis.text.y = ggplot2::element_text()) + 
    ggplot2::theme(axis.title.x = ggplot2::element_text()) +
    ggplot2::scale_x_discrete() +  # Explicitly use discrete scale
    ggplot2::scale_y_discrete()    # Explicitly use discrete scale

    p <- p + ggplot2::theme(axis.text.x = ggplot2::element_blank(), 
                          axis.title.x = ggplot2::element_blank())
    final_plot <- patchwork::wrap_plots(p, leg, ncol = 1, heights = c(size_ratio, 1))
  
  return(final_plot)
}