Cutoff Line Clusters Dark

import matplotlib.pyplot as plt
import numpy as np
from scipy.cluster.hierarchy import dendrogram, linkage, fcluster, set_link_color_palette
from matplotlib.patches import Polygon

np.random.seed(321)

labels = ['P' + str(i) for i in range(1, 15)]
data = np.random.rand(len(labels), 6) * 70
Z = linkage(data, method='ward')

fig, ax = plt.subplots(figsize=(14, 8), facecolor='#0a0a0f')
ax.set_facecolor('#0a0a0f')

cutoff = 0.6 * max(Z[:,2])

set_link_color_palette(['#F5276C', '#27D3F5', '#6CF527', '#F5B027', '#5314E6'])

dn = dendrogram(Z, labels=labels, leaf_rotation=45, leaf_font_size=10,
                color_threshold=cutoff, above_threshold_color='#555555', ax=ax)

# Add cutoff line with glow
for lw, alpha in [(8, 0.1), (4, 0.3), (2, 0.8)]:
    ax.axhline(y=cutoff, color='#F5276C', linewidth=lw, alpha=alpha, linestyle='--' if lw == 2 else '-')

# Shade area below cutoff
ax.fill_between([ax.get_xlim()[0], ax.get_xlim()[1]], 0, cutoff, 
                color='#27D3F5', alpha=0.08, zorder=0)

ax.annotate('Cluster Cutoff', xy=(ax.get_xlim()[1]-10, cutoff), 
            color='#F5276C', fontsize=10, va='bottom')

ax.set_title('Dendrogram with Cluster Cutoff Threshold', fontsize=15, 
             color='white', fontweight='bold', pad=20)
ax.set_xlabel('Points', fontsize=11, color='#888888')
ax.set_ylabel('Ward Distance', fontsize=11, color='#888888')
ax.tick_params(axis='both', colors='#888888', labelsize=10)
ax.spines['top'].set_visible(False)
ax.spines['right'].set_visible(False)
ax.spines['left'].set_color('#333333')
ax.spines['bottom'].set_color('#333333')

plt.tight_layout()
plt.show()