Add a warning if tickmarks outside axis

Fixes #2990

python/tests/test_drawing.py

@@ -2220,6 +2220,25 @@ def test_y_axis_noticks(self):
         assert svg_no_css.count("y-axis") == 1
         assert svg_no_css.count('"tick"') == 0
 
+    def test_y_axis_tick_warning(sefl, caplog):
+        tree = msprime.simulate(4, random_seed=2).first()
+        upper = int(tree.time(tree.root))
+        with caplog.at_level(logging.WARNING):
+            tree.draw_svg(
+                y_axis=True,
+                y_label="Time",
+                y_ticks={upper + 100: "above", upper / 3: "inside"},
+            )
+            assert (
+                f"Ticks {{{upper+100}: 'above'}} lie outside the plotted axis"
+                in caplog.text
+            )
+        with caplog.at_level(logging.WARNING):
+            tree.draw_svg(
+                y_axis=True, y_label="Time", y_ticks={upper / 2: "inside", -1: "below"}
+            )
+            assert "Ticks {-1: 'below'} lie outside the plotted axis" in caplog.text
+
     def test_symbol_size(self):
         tree = msprime.simulate(4, random_seed=2, mutation_rate=8).first()
         sz = 24

python/tskit/drawing.py

@@ -25,6 +25,7 @@
 """
 import collections
 import itertools
+import logging
 import math
 import numbers
 import operator
@@ -919,12 +920,13 @@ def draw_y_axis(
         if self.y_axis:
             y_axis.add(dwg.line((x, rnd(lower)), (x, rnd(upper)), class_="ax-line"))
             ticks_group = y_axis.add(dwg.g(class_="ticks"))
+            tick_outside_axis = {}
             for y, label in ticks.items():
+                y_pos = self.timescaling.transform(y)
+                if y_pos > lower or y_pos < upper:  # nb lower > upper in SVG coords
+                    tick_outside_axis[y] = label
                 tick = ticks_group.add(
-                    dwg.g(
-                        class_="tick",
-                        transform=f"translate({x} {rnd(self.timescaling.transform(y))})",
-                    )
+                    dwg.g(class_="tick", transform=f"translate({x} {rnd(y_pos)})")
                 )
                 if gridlines:
                     tick.add(
@@ -941,6 +943,10 @@ def draw_y_axis(
                     class_="lab",
                     text_anchor="end",
                 )
+            if len(tick_outside_axis) > 0:
+                logging.warning(
+                    f"Ticks {tick_outside_axis} lie outside the plotted axis"
+                )
 
     def shade_background(
         self,