Add the graph-interaction row and its fixture

Nothing exercised the maximized graph's tools on either platform, which
is how Android 1.2.1 shipped a crash in the pick handler. The new
fixture is a straight line y = 2x + 1 with pick outputs, so that picking,
the difference and slope read-out, the linear fit and pan/zoom all have
exact expected values, plus a graph over empty containers that has to
survive every gesture. The row is planned until iOS carries the tag;
Android implements it (T0 geometry on Robolectric, T1 chrome on the
emulator).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: Staacks

fixtures/views/README.md

@@ -1,7 +1,7 @@
 # View fixtures
 
 The fixture set behind the view-element suites (test-matrix rows
-`view-snapshots`, `graph-snapshots`, `view-behavior`): one experiment per
+`view-snapshots`, `graph-snapshots`, `view-behavior`, `graph-interaction`): one experiment per
 element family, every configuration worth a golden, all data fixed by
 container `init` values — loading one of these renders a deterministic
 screen with no sensors and no analysis.
@@ -12,6 +12,13 @@ screen with no sensors and no analysis.
 - `graphs-styles`, `graphs-axes`, `graphs-special` — the OpenGL-rendered
   graphs (row `graph-snapshots`, T1: emulator + PixelCopy on Android,
   GLKView.snapshot in the simulator on iOS).
+- `graphs-interaction` — behavior only, no goldens (row `graph-interaction`,
+  T1): a straight line y = 2x + 1 with pick outputs, so that picking a
+  point, dragging for the difference and slope, the linear fit and
+  pan/zoom all have exact expected values, plus a graph over empty
+  containers that has to survive every one of those gestures (the
+  1.2.1 crash). The touch geometry itself runs at T0 on Android
+  (GraphView is a plain canvas view; only the curve needs GL).
 - `init-vs-default` — behavior only, no goldens: every input control once
   over a container that already holds a value and once over an empty one.
   The spec says a default fills an EMPTY buffer and never overwrites one,

fixtures/views/graphs-interaction.phyphox

@@ -0,0 +1,26 @@
+<phyphox xmlns="http://phyphox.org/xml" version="1.20" locale="en">
+    <title>fixture: graph interaction</title>
+    <category>View fixtures</category>
+    <description>The maximized graph's tools on fixed data: a straight line (y = 2x + 1) to pick, drag, pan, zoom and fit, with pick outputs so that a pick has a buffer effect, and a graph over empty containers, which has to survive every gesture.</description>
+    <data-containers>
+        <container size="9" init="0,1,2,3,4,5,6,7,8">x</container>
+        <container size="9" init="1,3,5,7,9,11,13,15,17">y</container>
+        <container size="100">nothing</container>
+        <container size="1">picked_x</container>
+        <container size="1">picked_y</container>
+    </data-containers>
+    <views>
+        <view label="Interaction">
+            <graph label="line" labelX="x" unitX="s" labelY="y" unitY="m" pickLabel="Pick">
+                <input axis="x">x</input>
+                <input axis="y">y</input>
+                <output axis="x" label="Pick x">picked_x</output>
+                <output axis="y" label="Pick y">picked_y</output>
+            </graph>
+            <graph label="empty" labelX="x" labelY="y">
+                <input axis="x">nothing</input>
+                <input axis="y">nothing</input>
+            </graph>
+        </view>
+    </views>
+</phyphox>

test-matrix.yml

@@ -202,6 +202,25 @@ tests:
       Android: emulator + PixelCopy; iOS: GLKView snapshot in the
       simulator. Goldens live in the app repos.
 
+  - id: graph-interaction
+    area: ui
+    tier: T1
+    platforms: [android, ios]
+    status: planned
+    description: >-
+      The maximized graph's tools over fixtures/views/graphs-interaction.phyphox:
+      maximizing and restoring a graph, picking a point (read-out values
+      and the pick outputs written to their containers, asserted through
+      the remote API), dragging for the difference and slope read-out,
+      the linear fit (a = 2, b = 1 on the fixture's line), panning and the
+      zoom reset - and every one of those gestures on a graph over empty
+      containers, which crashed Android 1.2.1 (a null buffer in the pick
+      handler). Android splits the row: the touch geometry (pick, drag,
+      pan, pinch, empty buffers) runs at T0 on Robolectric against
+      GraphView directly, the chrome around it (markers, read-outs, tools
+      menu, outputs) on the emulator. Added 2026-09-12 after the 1.2.1
+      crash review; iOS side pending (iOS-TODO.md).
+
   - id: view-behavior
     area: ui
     tier: T1