Add drag-and-snap tiling layout canvas to Plot Builder

Replace layout radio buttons (Stacked/Side-by-Side/Grid) with a visual
LayoutCanvas widget. Pane tiles snap to grid cells on drag-drop; dropping
on an occupied cell swaps positions. Ghost highlight shows snap target
during drag.

- PaneSpec gains row/col fields (None = unassigned; backward compat preserved)
- LayoutConfig gains "free" mode; strip_chart reads explicit positions
- PaneBlock loses ▲▼ move buttons; reordering is canvas-only
- QSS entries added for layoutTile, layoutTileDragging, layoutCanvasBar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/ui/strip_chart.py b/ui/strip_chart.py
index 523102d..d855038 100644
--- a/ui/strip_chart.py
+++ b/ui/strip_chart.py
@@ -95,12 +95,14 @@ class StripChartWidget(QWidget):
         n  = len(cfg.panes)
 
         # Determine row/col for each pane
-        if cfg.layout_mode == "sidebyside":
+        if cfg.layout_mode == "free":
+            positions = [(spec.row or 0, spec.col or 0) for spec in cfg.panes]
+        elif cfg.layout_mode == "sidebyside":
             positions = [(0, c) for c in range(n)]
         elif cfg.layout_mode == "grid":
             cols = max(1, cfg.grid_cols)
             positions = [(i // cols, i % cols) for i in range(n)]
-        else:  # stacked
+        else:  # stacked (legacy default)
             positions = [(r, 0) for r in range(n)]
 
         ref_plot = None
@@ -112,7 +114,7 @@ class StripChartWidget(QWidget):
             plot.getAxis("left").setStyle(tickFont=mf)
             plot.getAxis("bottom").setStyle(tickFont=mf)
 
-            # Hide bottom axis labels for all but bottom row in stacked
+            # Hide bottom axis labels for non-bottom panes in stacked legacy mode
             if cfg.layout_mode == "stacked" and idx < n - 1:
                 plot.getAxis("bottom").setStyle(showValues=False)
                 plot.getAxis("bottom").setHeight(0)
@@ -133,10 +135,9 @@ class StripChartWidget(QWidget):
             else:
                 plot.enableAutoRange(axis="y")
 
-            # Set column stretch (relative weight)
+            # Apply weight to row and column stretch
             self._gw.ci.layout.setColumnStretchFactor(col, spec.weight)
-            if cfg.layout_mode == "stacked":
-                self._gw.ci.layout.setRowStretchFactor(row, spec.weight)
+            self._gw.ci.layout.setRowStretchFactor(row, spec.weight)
 
             for tr in spec.traces:
                 if not tr.visible: continue