Reset the engine after a template builds, not before

js/app-library.js

@@ -225,16 +225,29 @@ class AppLibrary {
     }
   }
 
-  async _loadTemplate(t) {
-    if (!await this._confirmGuard(`Load "${t.name}"? Your current diagram will be replaced (Ctrl+Z to undo).`, 'Load template')) return;
+  // Swap the current diagram for a starter template. Shared with _loadDemo so
+  // the two cannot drift; each caller adds its own last step.
+  _installTemplate(t) {
     const prev = this._snapshot();
     this._clearAll();
     t.load();
     this.diagram.meta.name = t.name;
     this.diagram.meta.description = t.desc;
     this._applyMeta();
+    // Reset AFTER the template has built its nodes, not before. _clearAll resets
+    // too, but that runs against an empty diagram, so the step-0 baseline it
+    // records holds nothing. Leaving it that way meant the first point of every
+    // chart was a hole: run a freshly loaded template and the series began at
+    // step 1, so the starting amounts never plotted and spike attribution had no
+    // "from" value for the first step.
+    this.engine.reset();
     this._commitReplace(prev);
     this.renderer.fitView();
+  }
+
+  async _loadTemplate(t) {
+    if (!await this._confirmGuard(`Load "${t.name}"? Your current diagram will be replaced (Ctrl+Z to undo).`, 'Load template')) return;
+    this._installTemplate(t);
     this._hideModal('lib-overlay');
   }
 

js/app.js

@@ -850,14 +850,7 @@ class App {
   _loadDemo() {
     const t = this._templates[0]; // Predator & Prey
     if (!t) return;
-    const prev = this._snapshot();
-    this._clearAll();
-    t.load();
-    this.diagram.meta.name = t.name;
-    this.diagram.meta.description = t.desc;
-    this._applyMeta();
-    this._commitReplace(prev);
-    this.renderer.fitView();
+    this._installTemplate(t);
     this._renderProps();
   }
 

test/smoke.js

@@ -175,6 +175,37 @@ const URL = process.env.SMOKE_URL || 'http://localhost:8080/';
     if (n > 0) ok(`template "${name}" loaded (${n} nodes)`); else fail(`template "${name}" empty`);
   }
 
+  // Loading a template has to reset the engine AFTER the template builds its
+  // nodes. _clearAll resets too, but against an empty diagram, so the step-0
+  // baseline it writes is empty and every chart series opens on a hole: the
+  // starting amounts never plot and the curve begins at step 1. Both entry
+  // points (Library template, welcome demo) go through _installTemplate.
+  const step0 = await page.evaluate(async () => {
+    const out = {};
+    // Drive the real entry points, not the shared helper, so this test measures
+    // behaviour rather than the presence of a particular function.
+    for (const [label, load] of [['template', () => window.app._loadTemplate(window.app._templates[0])],
+                                 ['demo', () => window.app._loadDemo()]]) {
+      await load();
+      const e = window.app.engine, d = window.app.diagram;
+      const h0 = e.history[0];
+      const snap = h0 ? h0.snap : null;
+      const ids = snap ? Object.keys(snap) : [];
+      // Every node the template built must appear in the step-0 baseline.
+      const covered = d.nodes.size > 0 && [...d.nodes.keys()].every(id => snap && snap[id] !== undefined);
+      // And stepping must leave no hole at index 0 of a series.
+      const probe = [...d.nodes.values()][0];
+      for (let i = 0; i < 3; i++) e.doStep();
+      const series = e.history.map(h => (h.snap && h.snap[probe.id] !== undefined) ? h.snap[probe.id] : null);
+      out[label] = { nodes: d.nodes.size, step0Entries: ids.length, covered, firstIsNull: series[0] === null, len: series.length };
+    }
+    return out;
+  });
+  if (step0.template.covered && !step0.template.firstIsNull
+    && step0.demo.covered && !step0.demo.firstIsNull)
+    ok('template/demo load records a step-0 baseline, so charts start at step 0 instead of a hole');
+  else fail('template step-0 baseline: ' + JSON.stringify(step0));
+
   // Navigation: zoom controls step the scale and update the readout; fit-to-content
   // re-frames without error.
   const nav = await page.evaluate(() => {