diff --git a/doc/whats_new.rst b/doc/whats_new.rst
index d4dfe7472..41ad1a56b 100644
--- a/doc/whats_new.rst
+++ b/doc/whats_new.rst
@@ -15,6 +15,8 @@ Changelog
 
 Bug
 ~~~
+- Fix GUI over-plotting of loaded data where the app stalled and did not plot
+  RMSE, by `George Dang`_ in :gh:`869`
 
 API
 ~~~
diff --git a/hnn_core/gui/_viz_manager.py b/hnn_core/gui/_viz_manager.py
index 63b04d3d6..86e72b0e3 100644
--- a/hnn_core/gui/_viz_manager.py
+++ b/hnn_core/gui/_viz_manager.py
@@ -343,6 +343,20 @@ def _dynamic_rerender(fig):
     fig.tight_layout()
 
 
+def _avg_dipole_check(dpls):
+    """Check for averaged dipole, else average the trials"""
+    # Check if there is an averaged dipole already
+    if not dpls:
+        return None
+
+    avg_dpls = [d for d in dpls if d.nave > 1]
+    if avg_dpls:
+        dpl = avg_dpls[0]
+    else:
+        dpl = average_dipoles(dpls)
+    return dpl
+
+
 def _plot_on_axes(b, simulations_widget, widgets_plot_type,
                   data_widget,
                   spectrogram_colormap_selection, max_spectral_frequency,
@@ -427,7 +441,7 @@ def _plot_on_axes(b, simulations_widget, widgets_plot_type,
         t0 = 0.0
         tstop = dpls_processed[-1].times[-1]
         if len(dpls_processed) > 1:
-            dpl = average_dipoles(dpls_processed)
+            dpl = _avg_dipole_check(dpls_processed)
         else:
             dpl = dpls_processed
         rmse = _rmse(dpl, target_dpl_processed, t0, tstop)
@@ -983,8 +997,10 @@ def _simulate_edit_figure(self, fig_name, ax_name, simulation_name,
                 Type of visualization.
             preprocessing_config : dict
                 A dict of visualization preprocessing parameters. Allowed keys:
-                `dipole_smooth`, `dipole_scaling`, `max_spectral_frequency`,
-                `spectrogram_colormap_selection`. config could be empty: `{}`.
+                `dipole_smooth`, `dipole_scaling`,
+                `data_to_compare`, `data_smooth`, `data_scaling`
+                `max_spectral_frequency`, `spectrogram_colormap_selection`.
+                config could be empty: `{}`.
             operation : str
                 `"plot"` if you want to plot and `"clear"` if you want to
                 remove previously plotted visualizations.
@@ -1016,8 +1032,11 @@ def _simulate_edit_figure(self, fig_name, ax_name, simulation_name,
         config_name_idx = {
             "dipole_smooth": 2,
             "dipole_scaling": 3,
-            "max_spectral_frequency": 4,
-            "spectrogram_colormap_selection": 5,
+            "data_to_compare": 4,
+            "data_smooth": 5,
+            "data_scaling": 6,
+            "max_spectral_frequency": 7,
+            "spectrogram_colormap_selection": 8,
         }
         for conf_key, conf_val in preprocessing_config.items():
             assert conf_key in config_name_idx.keys()
diff --git a/hnn_core/tests/test_gui.py b/hnn_core/tests/test_gui.py
index c795b295a..d10968ca0 100644
--- a/hnn_core/tests/test_gui.py
+++ b/hnn_core/tests/test_gui.py
@@ -678,7 +678,7 @@ def test_gui_adaptive_spectrogram(setup_gui):
 
 
 def test_gui_visualization(setup_gui):
-    """ Tests updating a figure creates plots with data. """
+    """Tests updating a figure creates plots with data."""
 
     gui = setup_gui
     gui.run_button.click()
@@ -712,6 +712,44 @@ def test_gui_visualization(setup_gui):
     plt.close('all')
 
 
+def test_dipole_data_overlay(setup_gui):
+    """Tests dipole plot with a simulation and data overlay."""
+    gui = setup_gui
+
+    # Run simulation with 2 trials
+    gui.widget_ntrials.value = 2
+    gui.run_button.click()
+
+    # Load data
+    file_path = assets_path / 'test_default.csv'
+    gui._simulate_upload_data(file_path)
+
+    # Edit the figure with data overlay
+    figid = 1
+    figname = f'Figure {figid}'
+    axname = 'ax1'
+    gui._simulate_viz_action("edit_figure", figname,
+                             axname, 'default', 'current dipole', {}, 'clear')
+    gui._simulate_viz_action("edit_figure", figname,
+                             axname, 'default', 'current dipole',
+                             {'data_to_compare': 'test_default'},
+                             'plot')
+    ax = gui.viz_manager.figs[figid].axes[1]
+
+    # Check number of lines
+    # 2 trials, 1 average, 2 data (data is over-plotted twice for some reason)
+    # But it only appears in the legend once.
+    assert len(ax.lines) == 5
+    assert len(ax.legend_.texts) == 2
+    assert ax.legend_.texts[0]._text == 'default: average'
+    assert ax.legend_.texts[1]._text == 'test_default'
+
+    # Check RMSE is printed
+    assert 'RMSE(default, test_default):' in ax.texts[0]._text
+
+    plt.close('all')
+
+
 def test_unlink_relink_widget():
     """Tests the unlinking and relinking of widgets decorator."""