fix(engine): make compile-snapshot test platform-independent

The snapshot was storing timezone-local Date objects and raw float
positions for time-scale ticks. macOS runs CDT/PDT (UTC-5/6) so D3
produces midnight-local dates like T05:00:00.000Z, while Linux CI runs
UTC so the same ticks land at T00:00:00.000Z. The fractional position
difference from the date shift was also enough to nudge fitContinuousTicks
onto a different candidate, producing an extra tick on Linux.

Fix: serializeLayout now normalizes tick Date values to YYYY-MM-DD UTC
strings and rounds positions to 2 decimal places before snapshotting.
Both changes are purely presentational -- the snapshot still asserts the
right labels and approximate positions.

Author: rileyhilliard

packages/engine/src/__tests__/__snapshots__/compile-snapshot.test.ts.snap

@@ -79,17 +79,17 @@ exports[`compileChart snapshot (Step 7 oracle) > clipped-domain bar chart (data
       "ticks": [
         {
           "label": "0",
-          "position": 35.269999999999996,
+          "position": 35.27,
           "value": 0,
         },
         {
           "label": "20",
-          "position": 251.96200000000002,
+          "position": 251.96,
           "value": 20,
         },
         {
           "label": "40",
-          "position": 468.654,
+          "position": 468.65,
           "value": 40,
         },
       ],
@@ -145,7 +145,7 @@ exports[`compileChart snapshot (Step 7 oracle) > clipped-domain bar chart (data
       "ticks": [
         {
           "label": "A",
-          "position": 297.0761194029851,
+          "position": 297.08,
           "value": "A",
         },
         {
@@ -155,7 +155,7 @@ exports[`compileChart snapshot (Step 7 oracle) > clipped-domain bar chart (data
         },
         {
           "label": "C",
-          "position": 119.52388059701492,
+          "position": 119.52,
           "value": "C",
         },
       ],
@@ -597,23 +597,23 @@ exports[`compileChart snapshot (Step 7 oracle) > legend-heavy multi-series line
       "ticks": [
         {
           "label": "2020",
-          "position": 42.99201707650273,
-          "value": 2020-01-01T06:00:00.000Z,
+          "position": 42.99,
+          "value": "2020-01-01",
         },
         {
           "label": "Apr 2020",
-          "position": 207.45089674408015,
-          "value": 2020-04-01T05:00:00.000Z,
+          "position": 207.45,
+          "value": "2020-04-01",
         },
         {
           "label": "Jul 2020",
-          "position": 371.98511259107465,
-          "value": 2020-07-01T05:00:00.000Z,
+          "position": 371.99,
+          "value": "2020-07-01",
         },
         {
           "label": "Oct 2020",
-          "position": 538.3273967440801,
-          "value": 2020-10-01T05:00:00.000Z,
+          "position": 538.33,
+          "value": "2020-10-01",
         },
       ],
       "titlePadding": undefined,
@@ -697,12 +697,12 @@ exports[`compileChart snapshot (Step 7 oracle) > legend-heavy multi-series line
         },
         {
           "label": "5",
-          "position": 370.04999999999995,
+          "position": 370.05,
           "value": 5,
         },
         {
           "label": "10",
-          "position": 329.69999999999993,
+          "position": 329.7,
           "value": 10,
         },
         {
@@ -717,17 +717,17 @@ exports[`compileChart snapshot (Step 7 oracle) > legend-heavy multi-series line
         },
         {
           "label": "25",
-          "position": 208.64999999999998,
+          "position": 208.65,
           "value": 25,
         },
         {
           "label": "30",
-          "position": 168.29999999999998,
+          "position": 168.3,
           "value": 30,
         },
         {
           "label": "35",
-          "position": 127.94999999999999,
+          "position": 127.95,
           "value": 35,
         },
         {
@@ -1485,22 +1485,22 @@ exports[`compileChart snapshot (Step 7 oracle) > watermarked column chart with g
       "ticks": [
         {
           "label": "A",
-          "position": 124.62862068965518,
+          "position": 124.63,
           "value": "A",
         },
         {
           "label": "B",
-          "position": 247.7228735632184,
+          "position": 247.72,
           "value": "B",
         },
         {
           "label": "C",
-          "position": 370.8171264367816,
+          "position": 370.82,
           "value": "C",
         },
         {
           "label": "D",
-          "position": 493.9113793103449,
+          "position": 493.91,
           "value": "D",
         },
       ],
@@ -1565,17 +1565,17 @@ exports[`compileChart snapshot (Step 7 oracle) > watermarked column chart with g
         },
         {
           "label": "20",
-          "position": 229.02857142857144,
+          "position": 229.03,
           "value": 20,
         },
         {
           "label": "40",
-          "position": 161.25714285714287,
+          "position": 161.26,
           "value": 40,
         },
         {
           "label": "60",
-          "position": 93.4857142857143,
+          "position": 93.49,
           "value": 60,
         },
       ],

packages/engine/src/__tests__/compile-snapshot.test.ts

@@ -14,11 +14,59 @@ import type { ChartLayout } from '@opendata-ai/openchart-core';
 import { describe, expect, it } from 'vitest';
 import { compileChart } from '../compile';
 
+/**
+ * Normalize a tick value for snapshot comparison. Date objects are converted
+ * to their UTC ISO date string (YYYY-MM-DD) so the snapshot doesn't encode
+ * the local timezone offset, which differs between macOS (CDT/PDT) and the
+ * Linux CI runner (UTC).
+ */
+function normalizeTickValue(value: unknown): unknown {
+  if (value instanceof Date) return value.toISOString().slice(0, 10);
+  return value;
+}
+
+/**
+ * Normalize a tick position to 2 decimal places. D3 time-scale positions are
+ * floating-point and shift slightly with timezone because the tick Date values
+ * differ by hours. Rounding eliminates that noise without losing signal.
+ */
+function normalizePosition(pos: unknown): unknown {
+  if (typeof pos === 'number') return Math.round(pos * 100) / 100;
+  return pos;
+}
+
+/** Normalize an axis tick array for platform-independent snapshot comparison. */
+function normalizeTicks(
+  ticks: Array<{ label?: string; value?: unknown; position?: unknown }>,
+): unknown[] {
+  return ticks.map((t) => ({
+    ...t,
+    value: normalizeTickValue(t.value),
+    position: normalizePosition(t.position),
+  }));
+}
+
+/** Normalize an axis object so tick values and positions are platform-stable. */
+function normalizeAxis(axis: Record<string, unknown> | undefined): unknown {
+  if (!axis) return axis;
+  const ticks = axis.ticks;
+  return {
+    ...axis,
+    ticks: Array.isArray(ticks)
+      ? normalizeTicks(ticks as Parameters<typeof normalizeTicks>[0])
+      : ticks,
+  };
+}
+
 /** Convert ChartLayout into a fully serializable shape for snapshot comparison. */
 function serializeLayout(layout: ChartLayout): Record<string, unknown> {
   const { tooltipDescriptors, measureText: _measure, ...rest } = layout;
+  const axes = rest.axes as
+    | { x?: Record<string, unknown>; y?: Record<string, unknown> }
+    | undefined;
   return {
     ...rest,
+    axes: axes ? { ...axes, x: normalizeAxis(axes.x), y: normalizeAxis(axes.y) } : axes,
     tooltipDescriptors: Array.from(tooltipDescriptors.entries()),
   };
 }