diff --git a/backend/prisma/migrations/20260601000000_add_experiment_plot_config/migration.sql b/backend/prisma/migrations/20260601000000_add_experiment_plot_config/migration.sql
new file mode 100644
index 0000000..01b6bb6
--- /dev/null
+++ b/backend/prisma/migrations/20260601000000_add_experiment_plot_config/migration.sql
@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "experiments" ADD COLUMN     "plot_config" JSONB;
diff --git a/backend/prisma/schema.prisma b/backend/prisma/schema.prisma
index 7c29b47..5eb5dbe 100644
--- a/backend/prisma/schema.prisma
+++ b/backend/prisma/schema.prisma
@@ -14,6 +14,7 @@ model Experiment {
   created_at            DateTime @default(now())
   template              Json     @default("[]")
   subject_info_template Json     @default("[]")
+  plot_config           Json?
   animals               Animal[]
 
   @@map("experiments")
diff --git a/backend/src/routes/experiments.js b/backend/src/routes/experiments.js
index 2581c29..01c6154 100644
--- a/backend/src/routes/experiments.js
+++ b/backend/src/routes/experiments.js
@@ -224,6 +224,29 @@ function validateSubjectTemplate(template) {
   return null;
 }
 
+// Upper bound on the stored UI-preference blob. Generous for the known fields;
+// guards against a client persisting an arbitrarily large object.
+const PLOT_CONFIG_MAX_BYTES = 16 * 1024;
+
+// The blob is intentionally schema-loose (clients may add display keys over time);
+// only hiddenSubjects is structurally validated since the backend reasons about it.
+function validatePlotConfig(config) {
+  if (config === null || typeof config !== 'object' || Array.isArray(config)) {
+    return 'plot_config must be an object';
+  }
+  // req.body is already JSON-parsed, so stringify cannot throw here.
+  if (Buffer.byteLength(JSON.stringify(config), 'utf8') > PLOT_CONFIG_MAX_BYTES) {
+    return 'plot_config too large';
+  }
+  if ('hiddenSubjects' in config) {
+    const hidden = config.hiddenSubjects;
+    if (!Array.isArray(hidden) || hidden.length > 500 || !hidden.every((x) => typeof x === 'string')) {
+      return 'hiddenSubjects invalid';
+    }
+  }
+  return null;
+}
+
 // ── Template endpoints ─────────────────────────────────────────────────────────
 
 // GET /api/experiments/:id/template
@@ -308,6 +331,24 @@ router.put('/:id/subject-template', async (req, res, next) => {
   } catch (err) { next(err); }
 });
 
+// PUT /api/experiments/:id/plot-config — persist UI display settings (no audit log)
+router.put('/:id/plot-config', async (req, res, next) => {
+  try {
+    const experiment = await prisma.experiment.findUnique({ where: { id: req.params.id } });
+    if (!experiment) return res.status(404).json({ error: 'Experiment not found' });
+
+    const config = req.body;
+    const validationError = validatePlotConfig(config);
+    if (validationError) return res.status(422).json({ error: validationError });
+
+    await prisma.experiment.update({
+      where: { id: req.params.id },
+      data: { plot_config: config },
+    });
+    res.json(config);
+  } catch (err) { next(err); }
+});
+
 // GET /api/experiments/:id/calendar?field=<fieldId|builtinKey>
 // Returns { field, days: { "YYYY-MM-DD": count } } where count = subjects with non-empty value that day.
 router.get('/:id/calendar', async (req, res, next) => {
diff --git a/backend/tests/experiments.test.js b/backend/tests/experiments.test.js
index b9927f8..be586fd 100644
--- a/backend/tests/experiments.test.js
+++ b/backend/tests/experiments.test.js
@@ -136,3 +136,49 @@ describe('GET /health', () => {
     expect(res.body.status).toBe('ok');
   });
 });
+
+describe('PUT /api/experiments/:id/plot-config', () => {
+  const CONFIG = { xField: '__days__', tickStep: 2, hiddenSubjects: ['a1', 'a2'], sidebarOpen: true };
+
+  it('saves a valid config and returns it without writing an audit log', async () => {
+    prisma.experiment.findUnique.mockResolvedValue(EXPERIMENT);
+    prisma.experiment.update.mockResolvedValue({ ...EXPERIMENT, plot_config: CONFIG });
+    const res = await request(app).put(`/api/experiments/${EXPERIMENT.id}/plot-config`).send(CONFIG);
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual(CONFIG);
+    expect(prisma.experiment.update).toHaveBeenCalledWith({
+      where: { id: EXPERIMENT.id },
+      data: { plot_config: CONFIG },
+    });
+    expect(prisma.auditLog.create).not.toHaveBeenCalled();
+  });
+
+  it('returns 404 when the experiment does not exist', async () => {
+    prisma.experiment.findUnique.mockResolvedValue(null);
+    const res = await request(app).put(`/api/experiments/${EXPERIMENT.id}/plot-config`).send(CONFIG);
+    expect(res.status).toBe(404);
+    expect(prisma.experiment.update).not.toHaveBeenCalled();
+  });
+
+  it('rejects a non-object body with 422', async () => {
+    prisma.experiment.findUnique.mockResolvedValue(EXPERIMENT);
+    const res = await request(app).put(`/api/experiments/${EXPERIMENT.id}/plot-config`).send([1, 2, 3]);
+    expect(res.status).toBe(422);
+    expect(prisma.experiment.update).not.toHaveBeenCalled();
+  });
+
+  it('rejects an invalid hiddenSubjects with 422', async () => {
+    prisma.experiment.findUnique.mockResolvedValue(EXPERIMENT);
+    const res = await request(app).put(`/api/experiments/${EXPERIMENT.id}/plot-config`).send({ hiddenSubjects: [1, 2] });
+    expect(res.status).toBe(422);
+    expect(prisma.experiment.update).not.toHaveBeenCalled();
+  });
+
+  it('rejects an oversized config with 422', async () => {
+    prisma.experiment.findUnique.mockResolvedValue(EXPERIMENT);
+    const big = { blob: 'x'.repeat(20000) };
+    const res = await request(app).put(`/api/experiments/${EXPERIMENT.id}/plot-config`).send(big);
+    expect(res.status).toBe(422);
+    expect(prisma.experiment.update).not.toHaveBeenCalled();
+  });
+});