diff --git a/tests/testthat/test-diagnostics-extra.R b/tests/testthat/test-diagnostics-extra.R
new file mode 100644
index 0000000..66e86f1
--- /dev/null
+++ b/tests/testthat/test-diagnostics-extra.R
@@ -0,0 +1,35 @@
+# Additional coverage tests for AIC / AICweights diagnostics methods
+library(testthat)
+library(EnvCpt)
+
+# Minimal envcpt-like object for exercising diagnostics methods directly
+fake_envcpt <- list(
+  summary = matrix(
+    c(10, 20,
+      1,  2),
+    nrow = 2,
+    byrow = TRUE
+  )
+)
+class(fake_envcpt) <- "envcpt"
+
+# AIC.envcpt should respect a user-supplied penalty value
+test_that("AIC.envcpt respects custom penalty k", {
+  out <- AIC(fake_envcpt, k = 3)
+  expect_equal(out, c(13, 26))
+})
+
+# The default AICweights method should return the documented fallback message
+test_that("AICweights.default returns fallback message", {
+  out <- AICweights(1:5)
+  expect_equal(out, "No default method created for S3 class AICweights.")
+})
+
+# AICweights.envcpt should return normalized weights derived from AIC values
+test_that("AICweights.envcpt returns normalized weights", {
+  w <- AICweights(fake_envcpt)
+  aic <- AIC(fake_envcpt)
+  expected <- exp(-0.5 * (aic - min(aic)))
+  expected <- expected / sum(expected)
+  expect_equal(w, expected)
+})
\ No newline at end of file
diff --git a/tests/testthat/test-plot-extra.R b/tests/testthat/test-plot-extra.R
new file mode 100644
index 0000000..227f2b4
--- /dev/null
+++ b/tests/testthat/test-plot-extra.R
@@ -0,0 +1,43 @@
+# Additional coverage tests for plot.envcpt argument handling
+library(testthat)
+library(EnvCpt)
+
+test_that("plot.envcpt errors when x is not envcpt", {
+  expect_error(
+    EnvCpt:::plot.envcpt(list()),
+    "x must be an object with class envcpt"
+  )
+})
+
+# These plotting branches are run locally / outside CRAN
+if (identical(Sys.getenv("NOT_CRAN"), "true")) {
+  set.seed(99)
+  x <- c(rnorm(40, 0, 1), rnorm(40, 3, 1))
+  out <- envcpt(x)
+# The function accepts the British spelling "colours", although it currently warns
+  test_that("plot.envcpt accepts colours alias but warns", {
+    expect_warning(plot(out, type = "aic", colours = rep("black", 12)))
+  })
+  
+  test_that("plot.envcpt errors when colors vector is too short", {
+    expect_error(
+      plot(out, type = "aic", colors = rep("black", 3)),
+      "colors must be a vector of length 12"
+    )
+  })
+  
+  test_that("plot.envcpt errors when a color is invalid", {
+    bad_cols <- c(rep("black", 11), "not_a_colour")
+    expect_error(
+      plot(out, type = "aic", colors = bad_cols),
+      "Atleast one of your colours is not resolvable by col2rgb."
+    )
+  })
+  
+  test_that("plot.envcpt errors on unsupported plot type", {
+    expect_error(
+      plot(out, type = "weird"),
+      "type supplied can only be 'aic', 'bic' or 'fit'"
+    )
+  })
+}