diff --git a/enzynet/visualization.py b/enzynet/visualization.py
index 052342c..9a28669 100644
--- a/enzynet/visualization.py
+++ b/enzynet/visualization.py
@@ -5,9 +5,10 @@
 
 # MIT License
 
-from typing import List, Optional, Text, Tuple
+from typing import Optional, Text, Tuple
 
 import numpy as np
+import matplotlib.colors as mcolors
 import matplotlib.pyplot as plt
 
 from enzynet import pdb
@@ -52,7 +53,11 @@ def plot_volume(vol: np.ndarray, pdb_id: Text, v_size: int, num: int,
     plt.rc('font', family='serif')
     fig = plt.figure(figsize=(4,4))
     ax = fig.gca(projection='3d')
-    ax.set_aspect('equal')
+
+    # Reproduces the functionality of ax.set_aspect('equal').
+    # Source: https://github.com/matplotlib/matplotlib/issues/17172#issuecomment-830139107
+    ax.set_box_aspect(
+        [ub - lb for lb, ub in (getattr(ax, f'get_{a}lim')() for a in 'xyz')])
 
     # Parameters.
     len_vol = vol.shape[0]
@@ -94,9 +99,9 @@ def plot_volume(vol: np.ndarray, pdb_id: Text, v_size: int, num: int,
     ax.zaxis._axinfo["grid"]['linewidth'] = 0.1
 
     # Change thickness of ticks.
-    ax.xaxis._axinfo["tick"]['linewidth'] = 0.1
-    ax.yaxis._axinfo["tick"]['linewidth'] = 0.1
-    ax.zaxis._axinfo["tick"]['linewidth'] = 0.1
+    ax.xaxis._axinfo["tick"]['linewidth'][True] = 0.1
+    ax.yaxis._axinfo["tick"]['linewidth'][True] = 0.1
+    ax.zaxis._axinfo["tick"]['linewidth'][True] = 0.1
 
     # Change tick placement.
     ax.xaxis._axinfo['tick']['inward_factor'] = 0
@@ -115,7 +120,7 @@ def plot_volume(vol: np.ndarray, pdb_id: Text, v_size: int, num: int,
 def cuboid_data(
         pos: Tuple[float, float, float],
         size: Tuple[int, int, int]=(1,1,1)
-) -> Tuple[List[List[float]], List[List[float]], List[List[float]]]:
+) -> Tuple[np.ndarray, np.ndarray, np.ndarray]:
     """Gets coordinates of cuboid."""
     # Gets the (left, outside, bottom) point.
     o = [a - b / 2 for a, b in zip(pos, size)]
@@ -132,15 +137,17 @@ def cuboid_data(
          [o[2], o[2], o[2] + h, o[2] + h, o[2]],
          [o[2], o[2], o[2] + h, o[2] + h, o[2]]]
 
-    return x, y, z
+    return np.array(x), np.array(y), np.array(z)
 
 
 def plot_cube_at(pos: Tuple[float, float, float] = (0,0,0),
                  ax: Optional[plt.gca] = None) -> None:
     """Plots a cube element at position pos."""
+    lightsource = mcolors.LightSource(azdeg=135, altdeg=0)
     if ax != None:
         X, Y, Z = cuboid_data(pos)
-        ax.plot_surface(X, Y, Z, color='g', rstride=1, cstride=1, alpha=1)
+        ax.plot_surface(X, Y, Z, color='g', rstride=1, cstride=1, alpha=1,
+                        lightsource=lightsource)
 
 
 def plot_cube_weights_at(pos: Tuple[float, float, float] = (0, 0, 0),
diff --git a/requirements.txt b/requirements.txt
index 62e9d69..5dc43c5 100755
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,7 +6,7 @@ keras
 tensorflow
 
 # Plot
-matplotlib
+matplotlib==3.4.1
 
 # Biology
 biopython