使用matplotlib fill_between填充区域时出现间隙问题求助

你使用Matplotlib的fill_between函数填充图表不同区域时出现间隙，开启interpolate=True又会导致填充形状变为非矩形。以下是你的代码：

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
x = np.arange(0, 4 * np.pi, 0.01)
y = np.sin(x)
ax.plot(x, y, color='black')

threshold = 0.75
ax.axhline(threshold, color='green', lw=2, alpha=0.7)
ax.fill_between(x, 0, 1, where=y > threshold,
                facecolor=(0.5,0,0,0.5), ec=None,transform=ax.get_xaxis_transform())

ax.fill_between(x, 0, 1, where=y <= threshold,
                facecolor=(0,0.5,0,0.5), ec=None, transform=ax.get_xaxis_transform())

放大后的间隙效果如图：

问题原因 #

间隙出现是因为fill_between基于离散的x点判断where条件，当相邻x点的y值刚好在阈值两侧时，填充区域的边界无法完全衔接；而interpolate=True会让填充区域跟随曲线的插值点变化，导致形状不再是垂直矩形。

解决方案 #

改用ax.axvspan填充满足条件的垂直区间，能保证填充区域是完整矩形且无间隙。具体步骤是先识别出y > threshold的连续x区间，再逐个填充：

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
x = np.arange(0, 4 * np.pi, 0.01)
y = np.sin(x)
ax.plot(x, y, color='black')

threshold = 0.75
ax.axhline(threshold, color='green', lw=2, alpha=0.7)

# 找出y > threshold的连续区间
mask = y > threshold
# 定位区间起止索引
indices = np.where(np.diff(mask.astype(int)) != 0)[0] + 1
# 处理首尾边界情况
if mask[0]:
    indices = np.insert(indices, 0, 0)
if mask[-1]:
    indices = np.append(indices, len(mask))

# 先填充绿色背景（y <= threshold的区域）
ax.fill_between(x, 0, 1, facecolor=(0,0.5,0,0.5), ec=None, transform=ax.get_xaxis_transform())

# 逐个填充红色垂直区间（y > threshold的区域）
for start_idx, end_idx in zip(indices[::2], indices[1::2]):
    x_start = x[start_idx]
    x_end = x[end_idx]
    ax.axvspan(x_start, x_end, ymin=0, ymax=1, facecolor=(0.5,0,0,0.5), ec=None, transform=ax.get_xaxis_transform())

plt.show()

说明 #

1. 先识别连续的目标区间，避免离散点判断带来的间隙问题；
2. 用ax.axvspan填充垂直区域，确保形状为完整矩形；
3. 先填充背景色再叠加目标区间，逻辑更简洁，无需处理反向区间。

内容的提问来源于stack exchange，提问作者Joe Roberts