diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..550dbc2
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,12 @@
+"""Shared pytest fixtures."""
+import pytest
+from PySide6.QtWidgets import QApplication
+
+
+@pytest.fixture(scope="session")
+def qapp():
+    """Provide a QApplication instance for the test session."""
+    app = QApplication.instance()
+    if app is None:
+        app = QApplication([])
+    yield app
diff --git a/tests/test_comparison.py b/tests/test_comparison.py
new file mode 100644
index 0000000..777bc1f
--- /dev/null
+++ b/tests/test_comparison.py
@@ -0,0 +1,63 @@
+"""Tests for src/pygui/backend/comparison.py."""
+
+import numpy as np
+from pygui.backend.comparison import compare_runs
+
+
+def test_identical_series():
+    a = [20.0, 30.0, 40.0, 50.0, 60.0]
+    result = compare_runs(a, a)
+    assert result["distance"] == 0.0
+    assert len(result["index_a"]) == len(a)
+    assert result["index_a"] == result["index_b"]
+
+
+def test_empty_series():
+    result = compare_runs([], [])
+    assert result["distance"] == float("inf")
+    assert result["index_a"] == []
+    assert result["index_b"] == []
+    assert result["warping_path"] == []
+
+
+def test_one_empty():
+    result = compare_runs([10.0, 20.0], [])
+    assert result["distance"] == float("inf")
+
+
+def test_aligned_series():
+    a = [10.0, 20.0, 30.0]
+    b = [15.0, 25.0, 35.0]
+    result = compare_runs(a, b)
+    assert result["distance"] >= 0.0
+    assert len(result["index_a"]) > 0
+    assert len(result["index_b"]) > 0
+
+
+def test_single_element():
+    result = compare_runs([42.0], [42.0])
+    assert result["distance"] == 0.0
+
+
+def test_different_lengths():
+    short = [10.0, 20.0]
+    long_ = [10.0, 15.0, 20.0, 25.0, 30.0]
+    result = compare_runs(short, long_)
+    assert result["distance"] >= 0.0
+    assert len(result["index_a"]) > 0
+    assert len(result["index_b"]) > 0
+    assert len(result["index_a"]) == len(result["index_b"])
+
+
+def test_return_structure():
+    a = [1.0, 2.0, 3.0, 4.0, 5.0]
+    b = [2.0, 3.0, 4.0, 5.0, 6.0]
+    result = compare_runs(a, b)
+    assert "distance" in result
+    assert "index_a" in result
+    assert "index_b" in result
+    assert "warping_path" in result
+    assert isinstance(result["distance"], float)
+    assert isinstance(result["index_a"], list)
+    assert isinstance(result["index_b"], list)
+    assert isinstance(result["warping_path"], list)
diff --git a/tests/test_graph_view.py b/tests/test_graph_view.py
new file mode 100644
index 0000000..31f4d5b
--- /dev/null
+++ b/tests/test_graph_view.py
@@ -0,0 +1,108 @@
+"""Tests for src/pygui/backend/visualization/graph_view.py."""
+import json
+from unittest.mock import MagicMock, PropertyMock, patch
+
+import pytest
+
+
+def _make_plot_widget():
+    """Create a mock PlotWidget with the interface GraphView expects."""
+    widget = MagicMock()
+    widget.removeItem = MagicMock()
+    widget.plot = MagicMock(return_value=MagicMock())
+    widget.setYRange = MagicMock()
+    widget.clear = MagicMock()
+    return widget
+
+
+class TestUpdateTrend:
+    def test_parses_valid_json(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        gv.updateTrend(json.dumps([20.0, 25.0, 30.0]))
+
+        assert gv._plot_widget.plot.called
+        gv._plot_widget.setYRange.assert_called_once()
+
+    def test_rejects_invalid_json(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        gv.updateTrend("not json")
+
+        assert not gv._plot_widget.plot.called
+
+    def test_rejects_none(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        gv.updateTrend(None)
+
+        assert not gv._plot_widget.plot.called
+
+    def test_empty_list(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        gv.updateTrend("[]")
+
+        # Empty list means nothing to plot, but clear IS called
+        assert gv._plot_widget.plot.called is False
+
+    def test_no_plot_widget(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        assert gv._plot_widget is None
+
+        # Should not crash
+        gv.updateTrend("[1, 2, 3]")
+
+
+class TestUpdateComparison:
+    def test_parses_valid_json(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        a = [10.0, 20.0, 30.0]
+        b = [12.0, 22.0, 32.0]
+        path = [[0, 0], [1, 1], [2, 2]]
+
+        gv.updateComparison(json.dumps(a), json.dumps(b), json.dumps(path))
+
+        assert gv._plot_widget.clear.called
+        assert gv._plot_widget.plot.call_count == 2
+
+    def test_rejects_invalid_json(self):
+        """clear() is called before parsing, so it always fires on valid widget."""
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        gv._plot_widget = _make_plot_widget()
+
+        gv.updateComparison("bad", json.dumps([1]), json.dumps([]))
+
+        # clear() is called before parsing — it always fires
+        assert gv._plot_widget.clear.called
+        # But plot() should NOT be called since parsing fails
+        assert not gv._plot_widget.plot.called
+
+    def test_no_plot_widget(self):
+        from pygui.backend.visualization.graph_view import GraphView
+
+        gv = GraphView()
+        assert gv._plot_widget is None
+
+        # Should not crash
+        gv.updateComparison("[1,2]", "[3,4]", "[[0,0]]")
diff --git a/tests/test_wafer_map_item.py b/tests/test_wafer_map_item.py
new file mode 100644
index 0000000..bc99450
--- /dev/null
+++ b/tests/test_wafer_map_item.py
@@ -0,0 +1,91 @@
+"""Tests for src/pygui/backend/visualization/wafer_map_item.py."""
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+# Use the session-scoped qapp fixture from conftest.py so Qt Property/Signal
+# descriptors work properly.
+pytestmark = pytest.mark.usefixtures("qapp")
+
+
+def test_export_image_valid_path():
+    """export_image saves a PNG when grabToImage succeeds."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+
+    mock_image = MagicMock()
+    mock_result = MagicMock()
+    mock_result.image.return_value = mock_image
+    item.grabToImage = MagicMock(return_value=mock_result)
+
+    assert item.export_image("/tmp/test_wafer.png") is True
+
+
+def test_export_image_empty_path():
+    """export_image returns False for empty/None path."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    assert item.export_image("") is False
+    assert item.export_image(None) is False
+
+
+def test_export_image_grab_fails():
+    """export_image returns False when grabToImage raises."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    item.grabToImage = MagicMock(side_effect=RuntimeError("grab failed"))
+
+    assert item.export_image("/tmp/test_wafer.png") is False
+
+
+def test_export_image_save_fails():
+    """export_image returns False when image.save raises."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    mock_image = MagicMock()
+    mock_image.save.side_effect = IOError("disk full")
+    mock_result = MagicMock()
+    mock_result.image.return_value = mock_image
+
+    item = WaferMapItem()
+    item.grabToImage = MagicMock(return_value=mock_result)
+
+    assert item.export_image("/tmp/test_wafer.png") is False
+
+
+def test_thickness_properties():
+    """Thickness data properties exist with correct defaults."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+
+    assert item._thickness_data == []
+    assert item._show_thickness is False
+
+
+def test_thickness_setter_updates_data():
+    """Setting thicknessData updates _thickness_data."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    item._rebuild_thickness = MagicMock()
+
+    item.thicknessData = [1.0, 2.0, 3.0]
+
+    assert item._thickness_data == [1.0, 2.0, 3.0]
+    assert item._rebuild_thickness.called
+
+
+def test_show_thickness_setter():
+    """Setting showThickness updates _show_thickness."""
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    assert item._show_thickness is False
+
+    item.showThickness = True
+    assert item._show_thickness is True