diff --git a/frontend/src/components/SubjectSidebar.jsx b/frontend/src/components/SubjectSidebar.jsx
new file mode 100644
index 0000000..2d836c5
--- /dev/null
+++ b/frontend/src/components/SubjectSidebar.jsx
@@ -0,0 +1,59 @@
+import React from 'react';
+import { groupLines, groupVisibilityState } from '../lib/crossSubjectChart';
+
+// Presentational panel listing subjects grouped by their group, with per-subject
+// and per-group visibility checkboxes. All state lives in the parent; this component
+// only renders and calls handlers.
+export function SubjectSidebar({
+  lines, hiddenSubjects, onToggleSubject, onToggleGroup, onShowAll, onHighlight,
+}) {
+  const groups = groupLines(lines);
+  const anyHidden = lines.some((l) => hiddenSubjects.has(l.id));
+
+  return (
+    <div className="w-[210px] shrink-0 border-l border-gray-100 pl-3">
+      <div className="flex items-center justify-between mb-2">
+        <span className="text-xs font-semibold text-gray-500 uppercase tracking-wide">Subjects</span>
+        {anyHidden && (
+          <button type="button" onClick={onShowAll} aria-label="Show all subjects"
+            className="text-xs text-indigo-500 hover:text-indigo-700">Show all</button>
+        )}
+      </div>
+      <div className="space-y-3 max-h-[320px] overflow-y-auto pr-1">
+        {groups.map(({ group, subjects }) => {
+          const ids = subjects.map((s) => s.id);
+          const state = groupVisibilityState(ids, hiddenSubjects);
+          return (
+            <div key={group}>
+              <label className="flex items-center gap-1.5 text-xs font-medium text-gray-600 cursor-pointer">
+                <input type="checkbox"
+                  checked={state === 'all'}
+                  ref={(el) => { if (el) el.indeterminate = state === 'some'; }}
+                  onChange={() => onToggleGroup(ids, state === 'all')}
+                  className="accent-indigo-500" />
+                <span className="truncate">{group}</span>
+                <span className="text-gray-400">({subjects.length})</span>
+              </label>
+              <div className="mt-1 space-y-0.5 pl-1">
+                {subjects.map((s) => (
+                  <label key={s.id}
+                    onMouseEnter={() => onHighlight(s.id)}
+                    onMouseLeave={() => onHighlight(null)}
+                    className="flex items-center gap-1.5 text-xs text-gray-600 cursor-pointer hover:bg-gray-50 rounded px-1 py-0.5">
+                    <input type="checkbox"
+                      checked={!hiddenSubjects.has(s.id)}
+                      onChange={() => onToggleSubject(s.id)}
+                      className="accent-indigo-500" />
+                    <span className="inline-block w-2.5 h-2.5 rounded-full shrink-0"
+                      style={{ backgroundColor: s.color }} />
+                    <span className="truncate">{s.name}</span>
+                  </label>
+                ))}
+              </div>
+            </div>
+          );
+        })}
+      </div>
+    </div>
+  );
+}
diff --git a/frontend/tests/SubjectSidebar.test.jsx b/frontend/tests/SubjectSidebar.test.jsx
new file mode 100644
index 0000000..069e123
--- /dev/null
+++ b/frontend/tests/SubjectSidebar.test.jsx
@@ -0,0 +1,104 @@
+import React from 'react';
+import { render, screen, fireEvent, within } from '@testing-library/react';
+import { SubjectSidebar } from '../src/components/SubjectSidebar';
+
+const LINES = [
+  { id: 'a1', name: 'Mouse-A1', group: 'Group A', color: '#111' },
+  { id: 'a2', name: 'Mouse-A2', group: 'Group A', color: '#222' },
+  { id: 'b1', name: 'Mouse-B1', group: 'Group B', color: '#333' },
+];
+
+function setup(overrides = {}) {
+  const props = {
+    lines: LINES,
+    hiddenSubjects: new Set(),
+    onToggleSubject: jest.fn(),
+    onToggleGroup: jest.fn(),
+    onShowAll: jest.fn(),
+    onHighlight: jest.fn(),
+    ...overrides,
+  };
+  render(<SubjectSidebar {...props} />);
+  return props;
+}
+
+it('renders each group header with its subject count', () => {
+  setup();
+  expect(screen.getByText('Group A')).toBeInTheDocument();
+  expect(screen.getByText('Group B')).toBeInTheDocument();
+  expect(screen.getByText('(2)')).toBeInTheDocument();
+  expect(screen.getByText('(1)')).toBeInTheDocument();
+});
+
+it('renders a subject checkbox per subject, checked when visible', () => {
+  setup({ hiddenSubjects: new Set(['a2']) });
+  expect(screen.getByLabelText('Mouse-A1').checked).toBe(true);
+  expect(screen.getByLabelText('Mouse-A2').checked).toBe(false);
+});
+
+it('calls onToggleSubject with the id when a subject checkbox is clicked', () => {
+  const { onToggleSubject } = setup();
+  fireEvent.click(screen.getByLabelText('Mouse-A1'));
+  expect(onToggleSubject).toHaveBeenCalledWith('a1');
+});
+
+it('shows the group checkbox as indeterminate when the group is mixed', () => {
+  setup({ hiddenSubjects: new Set(['a1']) });
+  const groupCheckbox = screen.getByLabelText(/Group A/);
+  expect(groupCheckbox.indeterminate).toBe(true);
+  expect(groupCheckbox.checked).toBe(false);
+});
+
+it('checks the group checkbox when all members are visible', () => {
+  setup();
+  expect(screen.getByLabelText(/Group A/).checked).toBe(true);
+});
+
+it('calls onToggleGroup with the group ids and hidden=true when an all-visible group is clicked', () => {
+  const { onToggleGroup } = setup();
+  fireEvent.click(screen.getByLabelText(/Group A/));
+  expect(onToggleGroup).toHaveBeenCalledWith(['a1', 'a2'], true);
+});
+
+it('shows the group checkbox unchecked and not indeterminate when all members are hidden', () => {
+  setup({ hiddenSubjects: new Set(['a1', 'a2']) });
+  const groupCheckbox = screen.getByLabelText(/Group A/);
+  expect(groupCheckbox.checked).toBe(false);
+  expect(groupCheckbox.indeterminate).toBe(false);
+});
+
+it('calls onToggleGroup with hidden=false when a fully-hidden group is clicked', () => {
+  const { onToggleGroup } = setup({ hiddenSubjects: new Set(['a1', 'a2']) });
+  fireEvent.click(screen.getByLabelText(/Group A/));
+  expect(onToggleGroup).toHaveBeenCalledWith(['a1', 'a2'], false);
+});
+
+it('hides the Show all link when nothing is hidden, shows it otherwise', () => {
+  const { rerender } = render(
+    <SubjectSidebar lines={LINES} hiddenSubjects={new Set()}
+      onToggleSubject={() => {}} onToggleGroup={() => {}}
+      onShowAll={() => {}} onHighlight={() => {}} />,
+  );
+  expect(screen.queryByText('Show all')).toBeNull();
+  rerender(
+    <SubjectSidebar lines={LINES} hiddenSubjects={new Set(['a1'])}
+      onToggleSubject={() => {}} onToggleGroup={() => {}}
+      onShowAll={() => {}} onHighlight={() => {}} />,
+  );
+  expect(screen.getByText('Show all')).toBeInTheDocument();
+});
+
+it('calls onShowAll when the Show all link is clicked', () => {
+  const { onShowAll } = setup({ hiddenSubjects: new Set(['a1']) });
+  fireEvent.click(screen.getByText('Show all'));
+  expect(onShowAll).toHaveBeenCalled();
+});
+
+it('calls onHighlight on subject row hover enter/leave', () => {
+  const { onHighlight } = setup();
+  const row = screen.getByLabelText('Mouse-A1').closest('label');
+  fireEvent.mouseEnter(row);
+  expect(onHighlight).toHaveBeenCalledWith('a1');
+  fireEvent.mouseLeave(row);
+  expect(onHighlight).toHaveBeenCalledWith(null);
+});