diff --git a/datascience/maps.py b/datascience/maps.py
index 652491b3..ef8c4d6e 100644
--- a/datascience/maps.py
+++ b/datascience/maps.py
@@ -527,25 +527,40 @@ def copy(self):
     def _folium_kwargs(self):
         attrs = self._attrs.copy()
         attrs['location'] = self.lat_lon
+
         icon_args = {k: attrs.pop(k) for k in attrs.keys() & {'color', 'marker_icon', 'clustered_marker', 'icon_angle', 'popup_width'}}
         if 'marker_icon' in icon_args:
             icon_args['icon'] = icon_args.pop('marker_icon')
+
         if 'color' in icon_args and icon_args['color'][0] == '#':
-            # Checks if color provided is a hex code instead; if it is, uses BeautifyIcon to create markers. 
-            # If statement does not check to see if color is an empty string.
-            icon_args['background_color'] = icon_args['border_color'] = icon_args.pop('color')
-            if icon_args['background_color'][1] == icon_args['background_color'][3] == icon_args['background_color'][5] == 'f':
-                icon_args['text_color'] = 'gray'
-            else:
-                icon_args['text_color'] = 'white'
+            # Use hex code -> BeautifyIcon
+            hex_color = icon_args.pop('color')
+            icon_args['background_color'] = hex_color
+            icon_args['border_color'] = hex_color
+
+            # Determine a visible text color
+            def text_color_for_bg(bg):
+                bg = bg.lstrip('#')
+                r, g, b = int(bg[0:2], 16), int(bg[2:4], 16), int(bg[4:6], 16)
+                brightness = (r*299 + g*587 + b*114)/1000
+                if brightness > 240:
+                    return 'gray'
+                elif brightness > 186:
+                    return 'black'
+                else:
+                    return 'white'
+
+            icon_args['textColor'] = text_color_for_bg(hex_color)
             icon_args['icon_shape'] = 'marker'
             if 'icon' not in icon_args:
                 icon_args['icon'] = 'circle'
             attrs['icon'] = BeautifyIcon(**icon_args)
         else:
             attrs['icon'] = folium.Icon(**icon_args)
+
         return attrs
 
+
     def geojson(self, feature_id):
         """GeoJSON representation of the marker as a point."""
         lat, lon = self.lat_lon
diff --git a/datascience/tables.py b/datascience/tables.py
index 51298c88..b7c5696f 100644
--- a/datascience/tables.py
+++ b/datascience/tables.py
@@ -3045,7 +3045,8 @@ def to_df(self):
             <class 'datascience.tables.Table'>
 
             >>> type(table_df)
-            <class 'pandas.core.frame.DataFrame'>
+            <class 'pandas.DataFrame'>
+
 
         """
         return pandas.DataFrame(self._columns)
diff --git a/tests/test_formats.py b/tests/test_formats.py
index 9659afd3..1124cdf4 100644
--- a/tests/test_formats.py
+++ b/tests/test_formats.py
@@ -152,3 +152,11 @@ def _mult_ten(v):
 
     func_formatter = formats.FunctionFormatter(_mult_ten)
     assert func_formatter.format_value(5) == '50'
+
+def test_distribution_formatter_negative():
+    """Test that DistributionFormatter raises assertion on negative values."""
+    import numpy as np
+    counts = np.array([9, 10, -18, 23])
+    formatter = ds.DistributionFormatter()
+    with pytest.raises(AssertionError):
+        formatter.convert_column(counts)
diff --git a/tests/test_maps.py b/tests/test_maps.py
index de9b8e63..d9cfaa42 100644
--- a/tests/test_maps.py
+++ b/tests/test_maps.py
@@ -44,6 +44,7 @@ def test_setup_map():
         'width': 960,
         'height': 500,
         'features': [],
+        'attr': 'Map data © OpenStreetMap contributors' 
     }
     """ Tests features as NumPy array. """
     kwargs2 = {
@@ -56,6 +57,7 @@ def test_setup_map():
             ds.Marker(51.514, -0.139), 
             ds.Marker(51.519, -0.132)
         ]),
+        'attr': 'Map data © OpenStreetMap contributors'
     }
     ds.Map(**kwargs1).show()
     ds.Map(**kwargs2).show()
@@ -163,12 +165,14 @@ def test_marker_copy():
 def test_background_color_condition_white():
     # Test the condition when the background color is white (all 'f' in the hex code)
     marker = ds.Marker(0, 0, color='#ffffff')
-    assert marker._folium_kwargs['icon'].options['textColor'], 'gray'
+    assert marker._folium_kwargs['icon'].options.get('textColor', None) == 'gray'
+
 
 def test_background_color_condition_not_white():
     # Test the condition when the background color is not white
     marker = ds.Marker(0, 0, color='#ff0000')
-    assert marker._folium_kwargs['icon'].options['textColor'], 'white'
+    assert marker._folium_kwargs['icon'].options.get('textColor', None) == 'white'
+
 
 def test_icon_args_icon_not_present():
     # Test when 'icon' key is not present in icon_args
diff --git a/tests/test_util.py b/tests/test_util.py
index 5066b1f2..83e8d7c8 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -148,3 +148,33 @@ def __len__(self):
             pass
     is_sequence = IsSequence()
     assert ds.is_non_string_iterable(is_sequence) == True
+
+
+def test_plot_normal_cdf_no_shading():
+    """Test plot_normal_cdf with no shading."""
+    ds.plot_normal_cdf()
+
+
+def test_plot_normal_cdf_right_bound():
+    """Test plot_normal_cdf with right boundary."""
+    ds.plot_normal_cdf(rbound=1.5)
+
+
+def test_plot_normal_cdf_left_bound():
+    """Test plot_normal_cdf with left boundary."""
+    ds.plot_normal_cdf(lbound=-1.5)
+
+
+def test_plot_normal_cdf_both_bounds():
+    """Test plot_normal_cdf with both boundaries."""
+    ds.plot_normal_cdf(lbound=-1.0, rbound=1.0)
+
+
+def test_plot_normal_cdf_custom_params():
+    """Test plot_normal_cdf with custom mean and standard deviation."""
+    ds.plot_normal_cdf(lbound=-2.0, rbound=2.0, mean=0, sd=1)
+
+
+def test_plot_cdf_area():
+    """Test plot_cdf_area (old name for plot_normal_cdf)."""
+    ds.plot_cdf_area(rbound=1.0)