diff --git a/R/persistence.R b/R/persistence.R
index 7a72da7..af03777 100644
--- a/R/persistence.R
+++ b/R/persistence.R
@@ -467,18 +467,10 @@ diagram_transform <- function(data, diagram) {
       x = (data$x + data$y) / 2,
       y = ifelse(
         is.infinite(data$x) & is.infinite(data$y),
+        # accommodate landscape horizons
         0,
         (data$y - data$x) / 2
       )
     )
   )
 }
-
-diagram_slope <- function(diagram) {
-  switch(
-    match.arg(diagram, c("flat", "diagonal", "landscape")),
-    flat = 0,
-    diagonal = 1,
-    landscape = 0
-  )
-}
diff --git a/tests/testthat/test-persistence.R b/tests/testthat/test-persistence.R
index 00ae202..c08090d 100644
--- a/tests/testthat/test-persistence.R
+++ b/tests/testthat/test-persistence.R
@@ -1,4 +1,33 @@
 
+test_that("`diagram_transform()` correctly transforms key points", {
+  d <- data.frame(
+    x = c(-Inf, 0, 0, exp(1), exp(1), pi,  Inf),
+    y = c(-Inf, 0, 1, exp(1), pi,     Inf, Inf)
+  )
+  
+  # diagonal transform (do nothing)
+  d_diag <- diagram_transform(d, "diagonal")
+  expect_identical(d, d_diag)
+  
+  # flat transform
+  d2 <- d
+  d2$y <- d$y - d$x
+  d_flat <- diagram_transform(d, "flat")
+  expect_identical(d2, d_flat)
+  
+  # landscape transform
+  d3 <- transform(
+    d,
+    x = (x + y) / 2,
+    y = (y - x) / 2
+  )
+  # accommodate landscape horizons
+  d3$y[c(1L, nrow(d3))] <- 0
+  d_land <- diagram_transform(d, "landscape")
+  expect_identical(d3, d_land)
+  
+})
+
 # ggplot object tests ----------------------------------------------------------
 
 # sample data set