diff --git a/README.md b/README.md
index e61b1dc..06861e1 100644
--- a/README.md
+++ b/README.md
@@ -107,6 +107,11 @@ can be a list of such colors, defining a colormap.
 
 **property `VolumeSlicer.axis`** (`int`): The axis to slice.
 
+**property `VolumeSlicer.extra_traces`**: A `dcc.Store` that can be used as an output to define
+additional traces to be shown in this slicer. The data must be
+a list of dictionaries, with each dict representing a raw trace
+object.
+
 **property `VolumeSlicer.graph`**: The `dcc.Graph` for this slicer. Use `graph.figure` to access the
 Plotly Figure object.
 
diff --git a/dash_slicer/slicer.py b/dash_slicer/slicer.py
index 6300bb8..66d4bee 100644
--- a/dash_slicer/slicer.py
+++ b/dash_slicer/slicer.py
@@ -238,6 +238,15 @@ def state(self):
         """
         return self._state
 
+    @property
+    def extra_traces(self):
+        """A `dcc.Store` that can be used as an output to define
+        additional traces to be shown in this slicer. The data must be
+        a list of dictionaries, with each dict representing a raw trace
+        object.
+        """
+        return self._extra_traces
+
     @property
     def overlay_data(self):
         """A `dcc.Store` containing the overlay data. The form of this
@@ -420,6 +429,9 @@ def _create_dash_components(self):
         # Store indicator traces for the slicer.
         self._indicator_traces = Store(id=self._subid("indicator-traces"), data=[])
 
+        # Store user traces for the slider.
+        self._extra_traces = Store(id=self._subid("extra-traces"), data=[])
+
         # A timer to apply a rate-limit between slider.value and index.data
         self._timer = Interval(id=self._subid("timer"), interval=100, disabled=True)
 
@@ -436,6 +448,7 @@ def _create_dash_components(self):
             self._server_data,
             self._img_traces,
             self._indicator_traces,
+            self._extra_traces,
             self._timer,
             self._state,
             self._setpos,
@@ -787,11 +800,12 @@ def _create_client_callbacks(self):
 
         app.clientside_callback(
             """
-        function update_figure(img_traces, indicators, info, ori_figure) {
+        function update_figure(img_traces, indicator_traces, extra_traces, info, ori_figure) {
             // Collect traces
             let traces = [];
             for (let trace of img_traces) { traces.push(trace); }
-            for (let trace of indicators) { if (trace.line.color) traces.push(trace); }
+            for (let trace of extra_traces) { traces.push(trace); }
+            for (let trace of indicator_traces) { if (trace.line.color) traces.push(trace); }
             // Update figure
             let figure = {...ori_figure};
             figure.data = traces;
@@ -802,6 +816,7 @@ def _create_client_callbacks(self):
             [
                 Input(self._img_traces.id, "data"),
                 Input(self._indicator_traces.id, "data"),
+                Input(self._extra_traces.id, "data"),
             ],
             [State(self._info.id, "data"), State(self._graph.id, "figure")],
         )
diff --git a/examples/threshold_contour.py b/examples/threshold_contour.py
new file mode 100644
index 0000000..ffda504
--- /dev/null
+++ b/examples/threshold_contour.py
@@ -0,0 +1,70 @@
+"""
+An example demonstrating adding traces.
+
+This shows a volume with contours overlaid on top. The `extra_traces`
+property is used to add scatter traces that represent the contours.
+"""
+
+import plotly
+import dash
+import dash_html_components as html
+import dash_core_components as dcc
+from dash.dependencies import Input, Output
+from dash_slicer import VolumeSlicer
+import imageio
+from skimage import measure
+
+
+app = dash.Dash(__name__, update_title=None)
+server = app.server
+
+vol = imageio.volread("imageio:stent.npz")
+mi, ma = vol.min(), vol.max()
+slicer = VolumeSlicer(app, vol)
+
+
+app.layout = html.Div(
+    [
+        slicer.graph,
+        slicer.slider,
+        dcc.Slider(
+            id="level-slider",
+            min=mi,
+            max=ma,
+            step=1,
+            value=mi + 0.2 * (ma - mi),
+        ),
+        *slicer.stores,
+    ]
+)
+
+
+@app.callback(
+    Output(slicer.extra_traces.id, "data"),
+    [Input("level-slider", "value"), Input(slicer.state.id, "data")],
+)
+def apply_levels(level, state):
+    if not state:
+        return dash.no_update
+    slice = vol[state["index"]]
+    contours = measure.find_contours(slice, level)
+    # Create a trace for each contour, each a different color
+    traces = []
+    for i, contour in enumerate(contours):
+        colors = plotly.colors.qualitative.D3
+        color = colors[i % len(colors)]
+        traces.append(
+            {
+                "type": "scatter",
+                "mode": "lines",
+                "line": {"color": color, "width": 3},
+                "x": contour[:, 1],
+                "y": contour[:, 0],
+            }
+        )
+    return traces
+
+
+if __name__ == "__main__":
+    # Note: dev_tools_props_check negatively affects the performance of VolumeSlicer
+    app.run_server(debug=True, dev_tools_props_check=False)
diff --git a/examples/threshold_overlay.py b/examples/threshold_overlay.py
index 997039f..5938950 100644
--- a/examples/threshold_overlay.py
+++ b/examples/threshold_overlay.py
@@ -22,13 +22,6 @@
 mi, ma = vol.min(), vol.max()
 slicer = VolumeSlicer(app, vol)
 
-slicer.graph.config.update(
-    modeBarButtonsToAdd=[
-        "drawclosedpath",
-        "eraseshape",
-    ]
-)
-
 
 app.layout = html.Div(
     [