Fix powder chart y-axis range and clarify planning workflow

.github/copilot-instructions.md

@@ -171,10 +171,23 @@ When asked to create a plan:
 - Apply the two-phase workflow (Phase 1 implementation, Phase 2
   verification) to non-trivial plans. Stop after Phase 1 and ask the
   user to review before starting Phase 2.
-- Every completed implementation step ends with a local commit following
-  the rules in **Commits**. Keep commits atomic, single-purpose, and
-  aligned with plan steps.
+- The plan must explicitly state that, when an AI agent follows it,
+  every completed Phase 1 implementation step must be staged with
+  explicit paths and committed locally before moving to the next
+  implementation step or the Phase 1 review gate. Follow the rules in
+  **Commits**. Keep commits atomic, single-purpose, and aligned with
+  plan steps.
+- If implementation uncovers a serious requirement, risk, design issue,
+  or scope change not covered by the plan, stop and ask the user for
+  clarification or approval before proceeding. Record the unresolved
+  issue in the plan when useful.
 - The plan should be easy to maintain while working: include concrete
   files likely to change, decisions already made, open questions,
   verification commands for Phase 2, and a short suggested commit
   message or branch name when useful.
+- End every plan with a "Suggested Pull Request" section containing a
+  short PR title and a brief end-user-oriented description. Keep this
+  section non-technical enough for scientists and other users to
+  understand the benefit. Update it during implementation if extra
+  approved changes become important enough to mention in the PR title or
+  description.

docs/dev/plan_powder-chart-y-range.md

@@ -0,0 +1,138 @@
+# Powder Chart Y-Range Fix Plan
+
+**Date:** 2026-05-06 **Status:** Phase 2 verified — complete
+
+---
+
+## 1. Goal
+
+Fix the Plotly composite powder measured-vs-calculated chart so the main
+intensity row is not anchored to zero. The y-axis range should be
+derived from all displayed main-row intensity series: measured
+(`Imeas`), calculated (`Icalc`), and background (`Ibkg`) when present.
+
+The intended display range is:
+
+```text
+lower = min(Imeas, Icalc, Ibkg) - margin
+upper = max(Imeas, Icalc, Ibkg) + margin
+```
+
+where `margin` is controlled by a dedicated constant of about 5% of the
+main intensity span. The lower bound must use `min - margin`, not
+`min + margin`, so the lowest displayed point remains visible with
+padding below it.
+
+---
+
+## 2. Current Findings
+
+- The affected code is `PlotlyPlotter._get_main_intensity_range()` in
+  `src/easydiffraction/display/plotters/plotly.py`.
+- It currently uses only `y_meas` and `y_calc`, then forces
+  `lower_limit = min(0.0, main_y_min)`. That explains positive powder
+  charts being truncated to a `0..max` range.
+- `PowderMeasVsCalcSpec` already carries optional `y_bkg`, and
+  `Plotter._plot_meas_vs_calc_data()` already filters
+  `pattern.intensity_bkg` into the spec for powder Bragg plots.
+- Existing Plotly unit tests assert the current `0.0..max` y-range in
+  the residual scale-match tests, so those expectations must change.
+- Repository memory notes confirm the background line is part of the
+  composite powder plot and should be considered display data.
+
+---
+
+## 3. Scope
+
+### In Scope
+
+- Add a module-level constant in
+  `src/easydiffraction/display/plotters/plotly.py`, likely
+  `MAIN_INTENSITY_RANGE_MARGIN_FRACTION = 0.05`.
+- Update `_get_main_intensity_range()` to compute min/max over `y_meas`,
+  `y_calc`, and non-empty `y_bkg` when present.
+- Apply symmetric visual padding outside the data range using the new
+  constant.
+- Preserve the existing empty-filtered-range behavior: empty required
+  series should still return a harmless fallback range.
+- Preserve residual scale matching by letting `_get_residual_limit()`
+  use the newly padded main range and the existing
+  `residual_height_fraction`, so the residual row remains adjusted to
+  the main row size as it is now.
+- Add/update focused unit tests for range calculation and affected
+  residual-scale expectations.
+
+### Out of Scope
+
+- No public plotting API changes.
+- No user-configurable y-axis margin in this step.
+- No changes to ASCII plotting unless a later review shows the same main
+  view problem exists there.
+- No refactor of plot layout, Bragg tick sizing, hover templates, or
+  facade routing.
+
+---
+
+## 4. Decisions
+
+- Use `min(Imeas, Icalc, Ibkg) - margin` for the lower y-axis bound and
+  `max(Imeas, Icalc, Ibkg) + margin` for the upper y-axis bound.
+- Keep the residual plot scaled to the main intensity row, preserving
+  the current matched-scale behavior after the main range gains padding.
+
+---
+
+## 5. Implementation Checklist
+
+- [ ] Create branch `feature/powder-chart-y-range` if requested.
+- [x] In `src/easydiffraction/display/plotters/plotly.py`, add the
+      dedicated 5% y-range margin constant near the other Plotly layout
+      constants.
+- [x] Update `_get_main_intensity_range()` so it includes background
+      intensity when available and uses the padded min/max range instead
+      of anchoring positive data to zero.
+- [x] Keep zero-span data explicit and stable, using a small fallback
+      range around the datum because a percentage margin is undefined.
+- [x] Confirm `_get_residual_limit()` continues to scale the residual
+      row from the updated main y-range and existing residual height
+      fraction.
+- [x] Stop after Phase 1 and request review before adding or running
+      tests, following the repo workflow.
+
+---
+
+## 6. Phase 2 Verification Checklist
+
+- [x] Add or update tests in
+      `tests/unit/easydiffraction/display/plotters/test_plotly.py` for:
+  - positive-only `Imeas`/`Icalc` data no longer starting at zero;
+  - `Ibkg` lowering or raising the main y-range when present;
+  - 5% padding on both ends of the main row;
+  - residual scale-match expectations after padding changes the main row
+    span;
+  - empty filtered arrays retaining the existing fallback behavior.
+- [x] Keep the existing facade propagation test in
+      `tests/unit/easydiffraction/display/test_plotting.py` unless the
+      implementation reveals a missing background handoff case.
+- [x] Run `pixi run fix`.
+- [x] Run `pixi run check` until clean.
+- [x] Run `pixi run unit-tests`.
+- [x] Run `pixi run integration-tests`.
+- [x] Run `pixi run script-tests`.
+
+---
+
+## 7. Likely Files
+
+- `src/easydiffraction/display/plotters/plotly.py`
+- `tests/unit/easydiffraction/display/plotters/test_plotly.py`
+- `tests/unit/easydiffraction/display/test_plotting.py` only if a
+  facade-level test gap is discovered during verification.
+
+---
+
+## 8. Suggested Commit Message
+
+```text
+Fix powder chart y-axis range
+```

src/easydiffraction/display/plotters/plotly.py

@@ -60,6 +60,7 @@
 BRAGG_TICK_MARKER_SIZE = 12
 BRAGG_TICK_MARKER_LINE_WIDTH = 1
 BRAGG_TICK_SYMBOL_HEIGHT_SCALE = 1.4
+MAIN_INTENSITY_RANGE_MARGIN_FRACTION = 0.05
 COMPOSITE_VERTICAL_SPACING = 0.03
 COMPOSITE_MARGIN_RIGHT = 30
 COMPOSITE_MARGIN_TOP = 40
@@ -910,12 +911,20 @@ def _get_main_intensity_range(cls, plot_spec: PowderMeasVsCalcSpec) -> tuple[flo
         if min(y_meas.size, y_calc.size) == 0:
             return 0.0, 1.0
 
-        main_y_min = float(min(np.min(y_meas), np.min(y_calc)))
-        main_y_max = float(max(np.max(y_meas), np.max(y_calc)))
-        lower_limit = min(0.0, main_y_min)
-        if main_y_max <= lower_limit:
-            return lower_limit - 1.0, lower_limit + 1.0
-        return lower_limit, main_y_max
+        main_series = [y_meas, y_calc]
+        if plot_spec.y_bkg is not None:
+            y_bkg = np.asarray(plot_spec.y_bkg)
+            if y_bkg.size > 0:
+                main_series.append(y_bkg)
+
+        main_y_min = float(min(np.min(series) for series in main_series))
+        main_y_max = float(max(np.max(series) for series in main_series))
+        main_y_range = main_y_max - main_y_min
+        if main_y_range > 0.0:
+            main_y_margin = main_y_range * MAIN_INTENSITY_RANGE_MARGIN_FRACTION
+            return main_y_min - main_y_margin, main_y_max + main_y_margin
+
+        return main_y_min - 1.0, main_y_max + 1.0
 
     @classmethod
     def _get_residual_limit(cls, plot_spec: PowderMeasVsCalcSpec) -> float:

tests/unit/easydiffraction/display/plotters/test_plotly.py

@@ -400,13 +400,40 @@ def fake_show_figure(self, fig):
     assert background_trace.mode == 'lines'
     assert background_trace.line.color == pp.DEFAULT_COLORS['bkg']
     assert background_trace.line.width == pp.BACKGROUND_LINE_WIDTH
+    raw_min = 1.5
+    raw_max = 12.0
+    raw_range = raw_max - raw_min
+    margin = raw_range * pp.MAIN_INTENSITY_RANGE_MARGIN_FRACTION
+    assert fig.layout.yaxis.range[0] == pytest.approx(raw_min - margin)
+    assert fig.layout.yaxis.range[1] == pytest.approx(raw_max + margin)
     assert meas_trace.legendrank < background_trace.legendrank < calc_trace.legendrank
     assert residual_trace.legendrank > calc_trace.legendrank
     for trace in (meas_trace, background_trace, calc_trace, residual_trace):
         assert 'Ibkg: %{customdata[1]' in trace.hovertemplate
         assert list(trace.customdata[0]) == pytest.approx([10.0, 1.5, 9.0, 1.0])
 
 
+def test_get_main_intensity_range_uses_unit_padding_for_flat_series():
+    from easydiffraction.display.plotters.base import PowderMeasVsCalcSpec
+    from easydiffraction.display.plotters.plotly import PlotlyPlotter
+
+    plot_spec = PowderMeasVsCalcSpec(
+        x=np.array([1.0]),
+        y_meas=np.array([5.0]),
+        y_calc=np.array([5.0]),
+        y_resid=None,
+        bragg_tick_sets=(),
+        axes_labels=['2θ (degree)', 'Intensity (arb. units)'],
+        title='Powder',
+        residual_height_fraction=0.25,
+        bragg_peaks_height_fraction=0.10,
+        height=None,
+        y_bkg=np.array([5.0]),
+    )
+
+    assert PlotlyPlotter._get_main_intensity_range(plot_spec) == pytest.approx((4.0, 6.0))
+
+
 def test_bragg_row_height_pixels_scale_linearly_with_phase_count():
     from easydiffraction.display.plotters.base import BraggTickSet
     from easydiffraction.display.plotters.base import PowderMeasVsCalcSpec
@@ -639,11 +666,17 @@ def fake_show_figure(self, fig):
     )
 
     fig = captured['fig']
-    expected_limit = 0.5 * (3600.0 - 0.0) * 0.25
+    raw_min = 180.0
+    raw_max = 3600.0
+    raw_range = raw_max - raw_min
+    margin = raw_range * pp.MAIN_INTENSITY_RANGE_MARGIN_FRACTION
+    expected_main_min = raw_min - margin
+    expected_main_max = raw_max + margin
+    expected_limit = 0.5 * (expected_main_max - expected_main_min) * 0.25
     assert fig.layout.yaxis2.scaleanchor == 'y'
     assert fig.layout.yaxis2.scaleratio == pytest.approx(1.0)
-    assert fig.layout.yaxis.range[0] == pytest.approx(0.0)
-    assert fig.layout.yaxis.range[1] == pytest.approx(3600.0)
+    assert fig.layout.yaxis.range[0] == pytest.approx(expected_main_min)
+    assert fig.layout.yaxis.range[1] == pytest.approx(expected_main_max)
     assert fig.layout.yaxis2.range[0] == pytest.approx(-expected_limit)
     assert fig.layout.yaxis2.range[1] == pytest.approx(expected_limit)
     plot_area_height = fig.layout.height - fig.layout.margin.t - fig.layout.margin.b
@@ -688,7 +721,11 @@ def fake_show_figure(self, fig):
     )
 
     fig = captured['fig']
-    expected_limit = 0.5 * (3600.0 - 0.0) * 0.25
+    raw_min = 180.0
+    raw_max = 3600.0
+    raw_range = raw_max - raw_min
+    margin = raw_range * pp.MAIN_INTENSITY_RANGE_MARGIN_FRACTION
+    expected_limit = 0.5 * ((raw_max + margin) - (raw_min - margin)) * 0.25
     assert fig.layout.yaxis2.range[0] == pytest.approx(-expected_limit)
     assert fig.layout.yaxis2.range[1] == pytest.approx(expected_limit)
     assert list(fig.layout.yaxis2.tickvals) == pytest.approx([-400.0, 0.0, 400.0])