Filled Stairs

import matplotlib.pyplot as plt
import numpy as np

COLORS = {
    'primary': '#6366F1',
    'background': '#FFFFFF',
    'text': '#1E293B',
    'text_muted': '#64748B',
    'grid': '#F1F5F9'
}

y = [2, 4, 6, 5, 7, 8, 6, 5, 7, 9, 8, 6]
edges = np.arange(len(y) + 1)

fig, ax = plt.subplots(figsize=(10, 6), dpi=100)
ax.set_facecolor(COLORS['background'])
fig.patch.set_facecolor(COLORS['background'])

# Clean gradient fill
ax.stairs(y, edges, fill=True, alpha=0.25, facecolor=COLORS['primary'], linewidth=0)
ax.stairs(y, edges, linewidth=2.5, color=COLORS['primary'])

# Peak marker
max_idx = np.argmax(y)
ax.scatter([max_idx + 0.5], [y[max_idx]], s=80, c=COLORS['primary'], edgecolors='white', linewidths=2, zorder=4)
ax.annotate(f'Peak: {y[max_idx]}', (max_idx + 0.5, y[max_idx]), 
            xytext=(8, 8), textcoords='offset points',
            fontsize=10, fontweight='bold', color=COLORS['primary'])

ax.spines['top'].set_visible(False)
ax.spines['right'].set_visible(False)
ax.spines['left'].set_color(COLORS['grid'])
ax.spines['bottom'].set_color(COLORS['grid'])
ax.yaxis.grid(True, color=COLORS['grid'], linewidth=1, zorder=0)
ax.set_axisbelow(True)
ax.tick_params(axis='both', colors=COLORS['text_muted'], labelsize=9, length=0, pad=8)
ax.set_xlim(0, 12)
ax.set_ylim(0, 11)
ax.set_xlabel('Step', fontsize=10, color=COLORS['text'], labelpad=10)
ax.set_ylabel('Value', fontsize=10, color=COLORS['text'], labelpad=10)

plt.tight_layout()
plt.show()