refactor: throw early when plotting empty graphs

Author: jolars

R/plot-core.R

@@ -150,6 +150,10 @@ S7::method(plot, caugi) <- function(
 ) {
   is_caugi(x, throw_error = TRUE)
 
+  if (is_empty_caugi(x)) {
+    stop("Cannot plot an empty graph (0 nodes).", call. = FALSE)
+  }
+
   stopifnot(
     is.null(asp) ||
       is.na(asp) ||

R/plot-grobs.R

@@ -55,9 +55,11 @@ make_tiers <- function(
   }
 
   # Normalize to 0-indexed
-  min_tier <- min(assignments)
-  if (min_tier > 0) {
-    assignments <- assignments - min_tier
+  if (length(assignments) > 0) {
+    min_tier <- min(assignments)
+    if (min_tier > 0) {
+      assignments <- assignments - min_tier
+    }
   }
 
   # Extract components

tests/testthat/test-plot.R

@@ -888,6 +888,15 @@ test_that("plot-grobs internal branches are covered", {
   expect_equal(length(c2$children), 3L)
 })
 
+test_that("plotting empty graph gives clear error", {
+  cg_empty <- caugi(class = "DAG")
+
+  expect_error(
+    plot(cg_empty),
+    "Cannot plot an empty graph \\(0 nodes\\)\\."
+  )
+})
+
 test_that("plot layout validation branch is covered", {
   cg <- caugi(
     from = LETTERS[1:7],