Fix dens(): honour p.rope and fail loudly on degenerate posteriors

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "baycomp_plotting"
-version = "1.2.0"
+version = "1.2.1"
 description = "Extra plotting functionality for baycomp's Bayesian classifier comparison posteriors."
 readme = "README.md"
 license = { file = "LICENSE" }

src/baycomp_plotting/plotting.py

@@ -90,6 +90,12 @@ def _add_posterior(ax, p, label: str, ls="-", color: str = Color.BLUE) -> None:
     upper bound) and ``zo`` (z-order counter, decremented for each posterior so
     earlier ones stay on top).
     """
+    if not (p.var > 0):
+        raise ValueError(
+            f"Cannot draw density: posterior variance is {p.var!r}. "
+            "The two classifiers likely produced identical scores; "
+            "the posterior is degenerate and has no density to plot."
+        )
     targs = (p.df, p.mean, np.sqrt(p.var))
     x = np.linspace(
         min(stats.t.ppf(0.005, *targs), -1.05 * p.rope),
@@ -205,8 +211,8 @@ def dens(p, label: str, ls="-", color: str = Color.BLUE):
 
     fig, ax = plt.subplots()
     fig.patch.set_alpha(0)
-    ax.axvline(0.01, c="darkorange", linewidth=2, zorder=101)
-    ax.axvline(-0.01, c="darkorange", linewidth=2, zorder=101)
+    ax.axvline(p.rope, c="darkorange", linewidth=2, zorder=101)
+    ax.axvline(-p.rope, c="darkorange", linewidth=2, zorder=101)
     ax.spines["right"].set_visible(False)
     ax.spines["top"].set_visible(False)
     ax.spines["bottom"].set_visible(False)

tests/test_plots.py

@@ -62,3 +62,42 @@ def test_tern_renders_three_vertex_labels(self, signed_rank_posterior):
         assert any("R" in t for t in text_contents)
         assert any("ROPE" in t for t in text_contents)
         plt.close(fig)
+
+
+class TestRopeAxvline:
+    """The two orange axvlines must reflect the posterior's ``rope`` parameter,
+    not a hardcoded value.
+    """
+
+    @pytest.mark.parametrize("rope", [0.001, 0.01, 0.05])
+    def test_axvlines_match_rope(self, rng, rope):
+        bc = pytest.importorskip("baycomp")
+        base = rng.normal(0.85, 0.015, 10)
+        x = base + rng.normal(0.0, 0.005, 10)
+        y = base + rng.normal(0.015, 0.005, 10)
+        posterior = bc.CorrelatedTTest(x, y, rope=rope, runs=1)
+
+        fig = bplt.dens(posterior, label="X")
+        ax = fig.axes[0]
+        vlines = sorted(
+            line.get_xdata()[0] for line in ax.lines
+            if line.get_xdata()[0] == line.get_xdata()[1]  # vertical line
+        )
+        # Two of those vertical lines should be at +/- rope
+        assert pytest.approx(rope, abs=1e-12) in vlines
+        assert pytest.approx(-rope, abs=1e-12) in vlines
+        plt.close(fig)
+
+
+class TestDegeneratePosterior:
+    """``dens`` should fail loudly when the posterior is degenerate (var=0),
+    instead of crashing on NaN axis limits inside matplotlib.
+    """
+
+    def test_var_zero_raises_value_error(self, rng):
+        bc = pytest.importorskip("baycomp")
+        identical = (rng.uniform(size=20) < 0.85).astype(float)
+        posterior = bc.CorrelatedTTest(identical, identical.copy(), rope=0.01, runs=1)
+
+        with pytest.raises(ValueError, match="degenerate"):
+            bplt.dens(posterior, label="X")