Fix the error band, and let a caller read back whether it is drawn

src/phosphor/trace_grid.py

@@ -338,6 +338,15 @@ def show_individual(self) -> bool:
     def show_mean(self) -> bool:
         return self._show_mean
 
+    @property
+    def show_error(self) -> bool:
+        """Whether the standard-deviation band is drawn.
+
+        Reads back the resolved value, not what was asked for: requesting a band
+        without statistics resolves to off, because there is no spread to draw.
+        """
+        return self._show_error
+
     @property
     def autoscale(self) -> bool:
         return self._autoscale
@@ -581,13 +590,20 @@ def _summary_positions(self) -> tuple[np.ndarray | None, np.ndarray | None]:
         return mean_pos, self._curve_positions(minmax_decimate(band, self._dec_plan))
 
     def _curve_positions(self, curve: np.ndarray) -> np.ndarray:
-        """One line per row of *curve*, laid into the cells."""
+        """One line per row of *curve*, laid into the cells.
+
+        *curve* holds a whole number of per-channel blocks -- one for a mean,
+        two for the lower and upper edges of a band. Each block is mapped on its
+        own, because the cell mapping is per channel and would otherwise be
+        asked to broadcast a block of channels against twice as many rows.
+        """
         n_lines, m = curve.shape[0], curve.shape[-1]
-        pos = np.empty((n_lines, m, 3), dtype=np.float32)
-        # The band is two curves per channel, so x tiles rather than broadcasts.
         reps = n_lines // self._n_ch
+        pos = np.empty((n_lines, m, 3), dtype=np.float32)
         pos[..., 0] = np.tile(self._x_line_dec, (reps, 1))
-        pos[..., 1] = self._map_y(curve)
+        for i in range(reps):
+            block = slice(i * self._n_ch, (i + 1) * self._n_ch)
+            pos[block, :, 1] = self._map_y(curve[block])
         pos[..., 2] = 0.0
         return pos
 

tests/test_trace_grid.py

@@ -33,8 +33,11 @@ def make_widget(n_ch=2, n_samples=4, history=3, **config_kwargs) -> TraceGridWid
     w._x_line_dec = np.tile(np.arange(n_samples, dtype=np.float32), (n_ch, 1))
     w._indiv_ml = w._mean_ml = w._error_ml = None
     w._graphics_version = -1
-    # _map_y is affine per channel; identity keeps these tests about layout.
-    w._map_y = lambda a: np.asarray(a, dtype=np.float32)
+    # The real _map_y, not a stub. It is per channel, and stubbing it is what
+    # let a band of 2 x n_ch rows reach it and raise on every frame while the
+    # tests stayed green.
+    w._rects = np.column_stack([np.zeros(n_ch), np.arange(n_ch, dtype=float), np.ones(n_ch)])
+    w._y_min, w._y_max = -1000.0, 1000.0
     return w
 
 
@@ -196,7 +199,9 @@ def test_the_mean_spans_more_waveforms_than_are_drawn():
         w._buffer.push(wave(v, n_ch=1))
 
     mean_pos, _ = w._summary_positions()
-    np.testing.assert_allclose(mean_pos[0, :, 1], 3.0)  # mean of 1..5, not of 4..5
+    # Compared through the cell mapping, since that is what the drawn y is.
+    expected = w._map_y(np.full((1, w._n_samples), 3.0, dtype=np.float32))  # 1..5, not 4..5
+    np.testing.assert_allclose(mean_pos[0, :, 1], expected[0], rtol=1e-6)
 
 
 # ---- graphics actually get created ------------------------------------------
@@ -286,7 +291,8 @@ def test_the_newest_waveform_reaches_the_graphic():
     w._refresh_lines()
 
     ys = w._indiv_ml.data[..., 1]
-    assert np.isclose(ys, 7.0).any(), "the value just pushed should be in the graphic"
+    expected = w._map_y(np.full((w._n_ch, w._n_samples), 7.0, dtype=np.float32))[0, 0]
+    assert np.isclose(ys, expected).any(), "the value just pushed should be in the graphic"
 
 
 def test_the_error_band_appears_once_there_is_a_spread():
@@ -316,3 +322,14 @@ def test_clearing_and_refilling_brings_the_graphics_back():
     w._buffer.push(wave(2.0))
     w._refresh_lines()
     assert w._indiv_ml is not None
+
+
+def test_show_error_reads_back_what_was_resolved_not_what_was_asked():
+    """A band without statistics is not a band. Callers persist this value, so
+    reading back the request rather than the resolution would restore a setting
+    that never took effect."""
+    w = make_widget(show_error=True, track_statistics=False)
+    assert w.show_error is False
+
+    w = make_widget(show_error=True)
+    assert w.show_error is True