diff --git a/pcmdi_metrics/graphics/parallel_coordinate_plot/parallel_coordinate_plot_lib.py b/pcmdi_metrics/graphics/parallel_coordinate_plot/parallel_coordinate_plot_lib.py
index c59450b62..8c37181a3 100644
--- a/pcmdi_metrics/graphics/parallel_coordinate_plot/parallel_coordinate_plot_lib.py
+++ b/pcmdi_metrics/graphics/parallel_coordinate_plot/parallel_coordinate_plot_lib.py
@@ -18,7 +18,7 @@ def parallel_coordinate_plot(
     models_to_highlight_by_line=True,
     models_to_highlight_colors=None,
     models_to_highlight_labels=None,
-    models_to_highlight_markers=['s', 'o', '^', '*'],
+    models_to_highlight_markers=["s", "o", "^", "*"],
     models_to_highlight_markers_size=10,
     fig=None,
     ax=None,
@@ -56,7 +56,7 @@ def parallel_coordinate_plot(
     - `data`: 2-d numpy array for metrics
     - `metric_names`: list, names of metrics for individual vertical axes (axis=1)
     - `model_names`: list, name of models for markers/lines (axis=0)
-    - `models_to_highlight`: list, default=None, List of models to highlight as lines or marker 
+    - `models_to_highlight`: list, default=None, List of models to highlight as lines or marker
     - `models_to_highlight_by_line`: bool, default=True, highlight as lines. If False, as marker
     - `models_to_highlight_colors`: list, default=None, List of colors for models to highlight as lines
     - `models_to_highlight_labels`: list, default=None, List of string labels for models to highlight as lines
@@ -242,14 +242,19 @@ def parallel_coordinate_plot(
                 label = models_to_highlight_labels[mh_index]
             else:
                 label = model
-            
+
             if models_to_highlight_by_line:
                 ax.plot(range(N), zs[j, :], "-", c=color, label=label, lw=3)
             else:
-                ax.plot(range(N), zs[j, :], models_to_highlight_markers[mh_index], 
-                        c=color, label=label, 
-                        markersize=models_to_highlight_markers_size)
-                
+                ax.plot(
+                    range(N),
+                    zs[j, :],
+                    models_to_highlight_markers[mh_index],
+                    c=color,
+                    label=label,
+                    markersize=models_to_highlight_markers_size,
+                )
+
             mh_index += 1
         else:
             if identify_all_models:
@@ -300,20 +305,28 @@ def parallel_coordinate_plot(
                     interpolate=False,
                     alpha=0.5,
                 )
-                
+
             if arrow_between_lines:
                 # Add vertical arrows
                 for xi, yi1, yi2 in zip(x, y1, y2):
-                    if (yi2 > yi1):
+                    if yi2 > yi1:
                         arrow_color = arrow_between_lines_colors[0]
-                    elif (yi2 < yi1):
+                    elif yi2 < yi1:
                         arrow_color = arrow_between_lines_colors[1]
                     else:
                         arrow_color = None
                     arrow_length = yi2 - yi1
-                    ax.arrow(xi, yi1, 0, arrow_length, color=arrow_color, 
-                            length_includes_head=True,
-                            alpha=arrow_alpha, width=0.05, head_width=0.15)
+                    ax.arrow(
+                        xi,
+                        yi1,
+                        0,
+                        arrow_length,
+                        color=arrow_color,
+                        length_includes_head=True,
+                        alpha=arrow_alpha,
+                        width=0.05,
+                        head_width=0.15,
+                    )
 
     ax.set_xlim(-0.5, N - 0.5)
     ax.set_xticks(range(N))