Add split-chain option to rank ecdf plots

Related to #333

R/mcmc-traces.R

@@ -485,6 +485,9 @@ mcmc_rank_hist <- function(x,
 #' @param plot_diff For `mcmc_rank_ecdf()`, a boolean specifying if the
 #' difference between the observed rank ECDFs and the theoretical expectation
 #' should be drawn instead of the unmodified rank ECDF plots.
+#' @param split_chains Logical indicating whether to split each chain into two parts.
+#'   If TRUE, each chain is split into first and second half with "_1" and "_2" suffixes.
+#'   Defaults to `FALSE`.
 #' @export
 mcmc_rank_ecdf <-
   function(x,
@@ -496,7 +499,8 @@ mcmc_rank_ecdf <-
            facet_args = list(),
            prob = 0.99,
            plot_diff = FALSE,
-           interpolate_adj = NULL) {
+           interpolate_adj = NULL,
+           split_chains = FALSE) {
   check_ignored_arguments(...,
     ok_args = c("K", "pit", "prob", "plot_diff", "interpolate_adj", "M")
   )
@@ -507,8 +511,28 @@ mcmc_rank_ecdf <-
     transformations = transformations,
     highlight = 1
   )
+
+  # Split chains if requested
+  if (split_chains) {
+    data$n_chains = data$n_chains/2
+    data$n_iterations = data$n_iterations/2
+    n_samples <- length(unique(data$iteration))
+    midpoint <- n_samples/2
+
+    data <- data %>%
+      group_by(.data$chain) %>%
+      mutate(
+        chain = ifelse(
+          iteration <= midpoint,
+          paste0(.data$chain, "_1"),
+          paste0(.data$chain, "_2")
+        )
+      ) %>%
+      ungroup()
+  }
+
   n_iter <- unique(data$n_iterations)
-  n_chain <- unique(data$n_chains)
+  n_chain <- length(unique(data$chain))
   n_param <- unique(data$n_parameters)
 
   x <- if (is.null(K)) {
@@ -561,7 +585,9 @@ mcmc_rank_ecdf <-
     group = .data$chain
   )
 
-  scale_color <- scale_color_manual("Chain", values = chain_colors(n_chain))
+  # Update legend title based on split_chains
+  legend_title <- if (split_chains) "Split Chains" else "Chain"
+  scale_color <- scale_color_manual(legend_title, values = chain_colors(n_chain))
 
   facet_call <- NULL
   if (n_param == 1) {