如何在Matplotlib中设置水平条形图边缘不扩展条形尺寸

背景 #

我正在开发一个基于Matplotlib的商业图表Python包，希望添加一个受IBCS启发的水平条形图功能。

问题描述 #

当前实现中，barheight是相对于类别间距的相对值，但linewidth是绝对值——边缘线会绘制在条形外部，导致实际占比空间变大。在大尺寸图表中，条形间仍有空白；但用相同参数创建小尺寸图表时，条形间空白消失，甚至出现前后条形互相遮挡的情况。

复现代码 #

import matplotlib.pyplot as plt
from random import *

linewidth = 2
barheight = 0.65

# 创建y坐标，预算条形略高于实际条形
y_budget = [y+0.1*barheight for y in range(10)]
y_actual = [y-0.1*barheight for y in range(10)]

# 生成随机条形长度
width_budget = [randint(1,100) for w in range(10)]
width_actual = [randint(1,100) for w in range(10)]

# 大尺寸图表
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(10, 6))
ax.barh(y=y_budget, width=width_budget, color='#FFFFFF', height=barheight, linewidth=linewidth, edgecolor='#404040')
ax.barh(y=y_actual, width=width_actual, color='#404040', height=barheight, linewidth=linewidth, edgecolor='#404040')
ax.set_yticks(y_actual, ["category "+str(i+1) for i in range(10)]);

# 小尺寸图表
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(10, 2))
ax.barh(y=y_budget, width=width_budget, color='#FFFFFF', height=barheight, linewidth=linewidth, edgecolor='#404040')
ax.barh(y=y_actual, width=width_actual, color='#404040', height=barheight, linewidth=linewidth, edgecolor='#404040')
ax.set_yticks(y_actual, ["category "+str(i+1) for i in range(10)]);

解决方案 #

方案1：将边缘线绘制在条形内部 #

Matplotlib的barh没有直接设置边缘线向内的参数，但可以通过双层条形模拟内部边框：底层绘制稍大的边框色条形，上层绘制稍小的填充色条形，实现边框在内部的视觉效果。

import matplotlib.pyplot as plt
from random import randint

barheight = 0.65
# 填充条形的高度比边框条形小，留出边框空间
border_height = barheight
fill_height = barheight - 0.2

# 数据准备
y_budget = [y+0.1*barheight for y in range(10)]
y_actual = [y-0.1*barheight for y in range(10)]
width_budget = [randint(1,100) for w in range(10)]
width_actual = [randint(1,100) for w in range(10)]

# 小尺寸图表测试
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(10, 2))

# 先绘制底层边框条形
ax.barh(y=y_budget, width=width_budget, color='#404040', height=border_height, linewidth=0)
ax.barh(y=y_actual, width=width_actual, color='#404040', height=border_height, linewidth=0)

# 再绘制上层填充条形
ax.barh(y=y_budget, width=width_budget, color='#FFFFFF', height=fill_height, linewidth=0)
ax.barh(y=y_actual, width=width_actual, color='#404040', height=fill_height, linewidth=0)

ax.set_yticks(y_actual, ["category "+str(i+1) for i in range(10)]);

方案2：让linewidth随图表尺寸相对变化 #

由于linewidth是像素单位，barheight是轴坐标单位，可通过图表像素与轴坐标的转换比例，动态计算适配的linewidth，保持其相对于barheight的比例固定。

import matplotlib.pyplot as plt
from random import randint

barheight = 0.65
# 设定linewidth占barheight的比例（例如5%）
linewidth_ratio = 0.05

# 数据准备
y_budget = [y+0.1*barheight for y in range(10)]
y_actual = [y-0.1*barheight for y in range(10)]
width_budget = [randint(1,100) for w in range(10)]
width_actual = [randint(1,100) for w in range(10)]

# 小尺寸图表测试
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(10, 2))

# 计算轴坐标与像素的转换比例
y_range = ax.get_ylim()[1] - ax.get_ylim()[0]
pixel_height = fig.dpi * fig.get_figheight()
pixels_per_y = pixel_height / y_range

# 动态计算适配的linewidth
linewidth = barheight * linewidth_ratio * pixels_per_y

# 绘制条形
ax.barh(y=y_budget, width=width_budget, color='#FFFFFF', height=barheight, linewidth=linewidth, edgecolor='#404040')
ax.barh(y=y_actual, width=width_actual, color='#404040', height=barheight, linewidth=linewidth, edgecolor='#404040')

# 修正绘制后轴范围变化带来的比例偏差
ax.set_ylim(-0.5, 9.5)
y_range = ax.get_ylim()[1] - ax.get_ylim()[0]
pixels_per_y = pixel_height / y_range
linewidth = barheight * linewidth_ratio * pixels_per_y

# 更新所有条形的linewidth
for patch in ax.patches:
    patch.set_linewidth(linewidth)

ax.set_yticks(y_actual, ["category "+str(i+1) for i in range(10)]);

方案3：手动绘制矩形精确控制边框 #

使用matplotlib.patches.Rectangle手动绘制条形，通过坐标转换将linewidth的像素单位转为轴坐标，精确调整矩形位置和尺寸，让边缘线完全在条形内部。

import matplotlib.pyplot as plt
from random import randint
from matplotlib.patches import Rectangle

barheight = 0.65
linewidth = 2  # 像素单位

# 数据准备
y_budget = [y+0.1*barheight for y in range(10)]
y_actual = [y-0.1*barheight for y in range(10)]
width_budget = [randint(1,100) for w in range(10)]
width_actual = [randint(1,100) for w in range(10)]

# 小尺寸图表测试
fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(10, 2))

# 将像素单位的linewidth转换为轴坐标单位
trans = ax.transData
pix_to_y = trans.inverted().transform((0, linewidth))[1] - trans.inverted().transform((0, 0))[1]

# 绘制预算条形（白填充+灰边框）
for y, w in zip(y_budget, width_budget):
    rect = Rectangle(
        (0, y - barheight/2 + pix_to_y), 
        w, 
        barheight - 2*pix_to_y,
        facecolor='#FFFFFF', 
        edgecolor='#404040', 
        linewidth=linewidth
    )
    ax.add_patch(rect)

# 绘制实际条形（灰填充+灰边框）
for y, w in zip(y_actual, width_actual):
    rect = Rectangle(
        (0, y - barheight/2 + pix_to_y), 
        w, 
        barheight - 2*pix_to_y,
        facecolor='#404040', 
        edgecolor='#404040', 
        linewidth=linewidth
    )
    ax.add_patch(rect)

# 设置轴范围
ax.set_ylim(-0.5, 9.5)
ax.set_xlim(0, max(max(width_budget), max(width_actual)) + 10)
ax.set_yticks(y_actual, ["category "+str(i+1) for i in range(10)]);

内容的提问来源于stack exchange，提问作者MarcelW1323