diff --git a/plot_auc_boxplots_by_chapter.R b/plot_auc_boxplots_by_chapter.R
index 47e85ee..8d3a0e4 100644
--- a/plot_auc_boxplots_by_chapter.R
+++ b/plot_auc_boxplots_by_chapter.R
@@ -15,7 +15,7 @@ args <- commandArgs(trailingOnly = TRUE)
 one_year_csv <- if (length(args) >= 1) args[1] else "model_comparison_auc_1year.csv"
 no_gap_csv   <- if (length(args) >= 2) args[2] else "model_comparison_auc_no_gap.csv"
 out_dir      <- if (length(args) >= 3) args[3] else "."
-orientation  <- if (length(args) >= 4) tolower(args[4]) else "horizontal"  # "horizontal" (flipped) or "vertical"
+orientation  <- if (length(args) >= 4) tolower(args[4]) else "vertical"  # "horizontal" (flipped) or "vertical"
 
 if (!dir.exists(out_dir)) {
   dir.create(out_dir, recursive = TRUE, showWarnings = FALSE)
@@ -38,6 +38,29 @@ get_chapter_col <- function(df) {
   return(NA_character_)
 }
 
+# Compute a deterministic chapter ordering using the ICD-10 chapter numeral prefix
+# e.g., "I. Infectious Diseases", "II. Neoplasms", ..., "XVII. ...", with a fallback for "Death" and unknowns
+compute_chapter_levels <- function(chapters) {
+  ch <- as.character(chapters)
+  roman_levels <- c(
+    "I","II","III","IV","V","VI","VII","VIII","IX","X",
+    "XI","XII","XIII","XIV","XV","XVI","XVII","XVIII","XIX","XX"
+  )
+  roman_map <- setNames(seq_along(roman_levels), roman_levels)
+  # Extract leading Roman numeral before a dot, like "XVI." -> "XVI"
+  roman <- toupper(gsub("^\\s*([IVXLCDM]+)\\..*$", "\\1", ch))
+  idx <- rep(NA_integer_, length(ch))
+  hit <- roman %in% names(roman_map)
+  idx[hit] <- roman_map[roman[hit]]
+  # Special-case Death at the end
+  idx[grepl("^\\s*Death\\b", ch, ignore.case = TRUE)] <- 99L
+  # Unknowns to the very end
+  idx[is.na(idx)] <- 100L
+  # Order chapters by idx, stable within same idx by appearance
+  o <- order(idx, match(ch, unique(ch)))
+  unique(ch[o])
+}
+
 # Build long-format data.frame with columns: chapter, model, auc
 # It will include any of the known model columns that exist in the input df
 build_long_df <- function(df) {
@@ -85,14 +108,8 @@ build_long_df <- function(df) {
 
 # Make the boxplot grouped by chapter
 make_boxplot <- function(long_df, title_text, flip = TRUE) {
-  # Order chapters by median AUC of Delphi if available, otherwise overall median
-  has_delphi <- any(long_df$model == "Delphi")
-  if (has_delphi) {
-    med <- aggregate(auc ~ chapter, data = subset(long_df, model == "Delphi"), median, na.rm = TRUE)
-  } else {
-    med <- aggregate(auc ~ chapter, data = long_df, median, na.rm = TRUE)
-  }
-  chap_levels <- med[order(med$auc, decreasing = TRUE), "chapter"]
+  # Order chapters by their ICD-10 chapter number prefix (Roman numerals)
+  chap_levels <- compute_chapter_levels(long_df$chapter)
   long_df$chapter <- factor(long_df$chapter, levels = chap_levels)
 
   p <- ggplot(long_df, aes(x = chapter, y = auc, fill = model)) +