图例

Axes.legend(
    *handles,                # 可选：显式指定进图例的 artist；不给就扫全图所有带 label 的
    *labels,                 # 可选：与 handles 一一对应的显示文字；不给就用 artist 自己的 label
    loc="best",              # ★ 位置：11 个字符串 / 0~10 整数 / (x, y) 比例坐标
    bbox_to_anchor=None,     # ★ 基准框：2 元组=一个点；4 元组=(x, y, w, h) 一块矩形
    ncols=1,                 # 列数（3.6 起正名，旧名 ncol 仍可用）
    fontsize=None,           # 条目字号；默认 None → 跟随 rcParams['legend.fontsize']（=10）
    title=None,              # 图例标题，可含 \n
    title_fontsize=None,     # 标题字号；★ 默认 None 时恒为 10，不跟着 fontsize 变
    frameon=True,            # 画不画外框
    fancybox=True,           # 圆角框（False → 方形直角）
    shadow=False,            # 外框投影
    framealpha=0.8,          # 外框透明度
    facecolor="inherit",     # 框底色；inherit = 跟随 axes.facecolor（通常白）
    edgecolor="0.8",         # 框边色
    borderpad=0.4,           # 框内边距（字号倍数）
    labelspacing=0.5,        # 条目之间的垂直间距
    handlelength=2.0,        # 图例里那段「示意线」的长度
    handleheight=0.7,        # 示意线的高度
    handletextpad=0.8,       # 示意线与文字的间距
    columnspacing=2.0,       # 多列时列与列的间距
    borderaxespad=0.5,       # 图例与 axes 边框之间的间距
    markerscale=1.0,         # 图例里 marker 的**线性**缩放
    numpoints=1,             # 折线条目上画几个 marker
    scatterpoints=1,         # 散点条目上画几个点
    handler_map=None,        # 自定义「一个 artist → 一条图例」的映射
    **kwargs                 # 其余 Text 属性（labelcolor / labelspacing 等）
)

import matplotlib.pyplot as plt
import numpy as np

x = np.arange(10)
fig, ax = plt.subplots()
ax.plot(x, x, label='线性')
ax.plot(x, x ** 1.5, label='超线性')
ax.plot(x, x ** 0.5, label='次线性')
ax.scatter(x[::3], (x ** 1.5)[::3], zorder=3, label='观测')

ax.legend()          # 一行就够：位置自动避让、条目自动取 label
plt.show()

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], [1, 2, 3], label='A')
lg = ax.legend(loc='best')
print(lg._loc)
# 输出: 0

fig.canvas.draw()
print(lg._loc, tuple(round(v, 1) for v in lg.get_window_extent().bounds))
# 输出: 0 (86.9, 390.3, 59.5, 25.1)    ← 位置变了，_loc 还是 0

import numpy as np
ax.plot(np.arange(5), np.arange(5), marker='o', label='带点')

res = {}
for n in (1, 3):
    lg = ax.legend(numpoints=n)
    line = lg.get_lines()[0]
    res[n] = [float(v) for v in line.get_xdata()], line.get_markevery()
print(res)
# 输出: {1: ([0.0, 10.0, 20.0], [1]), 3: ([3.0, 10.0, 17.0], None)}

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import matplotlib.patches as mpatches
import numpy as np

fig, ax = plt.subplots()
x = np.arange(10)
ax.plot(x, x, color='C0')

handles = [
    mlines.Line2D([], [], color='C0', lw=2, label='实测'),
    mlines.Line2D([], [], color='C1', lw=2, ls='--', label='预测'),
    mpatches.Patch(fc='C2', alpha=.5, label='置信区间'),
    mlines.Line2D([], [], color='none', marker='o', mfc='C3',
                  markersize=8, label='观测点'),
]
ax.legend(handles=handles, loc='upper left')
plt.show()

import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
from matplotlib.legend_handler import HandlerTuple

fig, ax = plt.subplots()
ax.bar([1], [3], color='#4C72B0')
ax.bar([2], [5], color='#DD8452')

p1 = mpatches.Patch(fc='#4C72B0')
p2 = mpatches.Patch(fc='#DD8452')
ax.legend([(p1, p2)], ['两段合并成一个条目'],
          handler_map={tuple: HandlerTuple(ndivide=None, pad=0.3)})
plt.show()

import matplotlib.pyplot as plt
import numpy as np
import matplotlib.patches as mpatches
from matplotlib.legend_handler import HandlerTuple

fig, axes = plt.subplots(1, 3, figsize=(13, 3.6))
x = np.arange(10)

# ① 图例放到轴右侧外面
ax = axes[0]
ax.plot(x, x, label='线性')
ax.plot(x, x ** 1.5, '--', label='超线性')
ax.plot(x, x ** 0.5, ':', label='次线性')
ax.legend(loc='upper left', bbox_to_anchor=(1.02, 1), borderaxespad=0)
ax.set_title('bbox_to_anchor 放轴外')

# ② 横贯整幅底部的图例条
ax = axes[1]
for i, c in enumerate(['C0', 'C1', 'C2', 'C3']):
    ax.plot(x, x * (i + 1) / 3, color=c, label=f'组 {i + 1}')
ax.legend(loc='upper center', bbox_to_anchor=(0, -.22, 1, .15), ncols=4,
          mode='expand', frameon=False, fontsize=9)
ax.set_title('mode="expand" 横贯条')

# ③ 用代理句柄 + HandlerTuple 合并条目
ax = axes[2]
b1 = ax.bar([1, 2], [3, 4], color='#4C72B0')
b2 = ax.bar([1, 2], [2, 1], bottom=[3, 4], color='#DD8452')
p1, p2 = mpatches.Patch(fc='#4C72B0'), mpatches.Patch(fc='#DD8452')
ax.legend([(p1, p2)], ['实际 + 计划'],
          handler_map={tuple: HandlerTuple(ndivide=None, pad=0.3)},
          loc='upper left', frameon=False)
ax.set_title('HandlerTuple 合并')

plt.tight_layout()
plt.show()