motplotlib图例案例1：通过多个legend完全控制图例显示顺序（指定按行排序 or 按列排序）

这个方法的核心，是手动的获得图中的handlers和labels，然后对它们进行切分和提取，最后分为几个legend进行显示。代码如下：

后来对下面的代码进行修改，通过handlers, labels = get_legend_handles_labels(axs=[axis])自动的获得handler。不再需要诸如handlers = [ax.scatter(range(10), [i * x for x in range(10)], label=f'Line {i}') for i in range(7)]之类的手动提取handler。源代码中这个依然进行了保留，是因为这个代码还是一个绘图过程

# -*- coding: utf-8 -*-
'''
@Time    : 2024/1/5 20:55  \n
@Author  : 月司   \n
@Email   : 815603884@qq.com \n
@File    : 图像图例的横向显示以及纵向显示.py   \n
@version : 1.0  \n
@Software: PyCharm  \n
@Desc    : 
'''

import matplotlib.pyplot as plt
import matplotlib.axes
from typing import List, Tuple, Any
#这个函数可以获得axis对象的已经绘制的artist中的所有的hander和labels，这个可以只给它一个axis参数。注意这个参数需要是列表类的。
from matplotlib.legend import _get_legend_handles_labels as get_legend_handles_labels

def custom_legend_layout(axis: matplotlib.axes.Axes,
                         handlers: List[Any]=None,
                         labels: List[str]=None,
                         n_items: int = 3,
                         offset: float = 0.05,
                         vertical: bool = False,
                         loc: str = 'upper right',
                         first_bbox_to_anchor: Tuple[float, float] = (1, 1),
                         **kwargs) -> None:
    """
    A function to arrange legend items in a custom layout.

    :param axis: Axis object on which to place the legends.
    :param lines: List of line objects to include in the legends.
    :param labels: List of labels corresponding to the line objects.
    :param n_items: Number of items per row (if vertical=False) or column (if vertical=True).
    :param offset: Vertical offset between rows (or horizontal offset between columns if vertical=True).
    :param vertical: If True, legends are arranged vertically, otherwise horizontally.
    :param loc: Location anchor for all legends.
    :param first_bbox_to_anchor: :param first_bbox_to_anchor:  `~matplotlib.transforms.BboxBase` instance, Bbox anchor of the first legend.Bbox anchor of the first legend.
    :param kwargs: Additional keyword arguments to pass to the legend function.
    """
    if (handlers is None) != (labels is None):  # Check if only one of handlers or labels is provided
        raise ValueError("Both 'handlers' and 'labels' must be specified if one is provided.")

    if (handlers is None) and (labels is None): # get default handlers and labels from ax
        handlers,labels=get_legend_handles_labels(axs=[axis]) # note:  the param axs is list object

    # 确保n_items不为0，避免除以0的错误
    n_items = max(1, n_items)
    # 计算需要多少个图例
    n_legends = len(handlers) // n_items + (1 if len(handlers) % n_items else 0)

    # 计算每个图例的bbox_to_anchor
    for i in range(n_legends):
        start_idx = i * n_items
        end_idx = min(start_idx + n_items, len(handlers))
        legend_lines = handlers[start_idx:end_idx]
        legend_labels = labels[start_idx:end_idx]

        if vertical:
            # 对于垂直布局
            ncol = 1
            if i == 0:
                bbox_anchor = first_bbox_to_anchor
            else:
                # 计算后续图例的bbox_to_anchor
                bbox_anchor = (first_bbox_to_anchor[0] + i * offset, first_bbox_to_anchor[1])
        else:
            # 对于水平布局
            ncol = len(legend_lines)
            if i == 0:
                bbox_anchor = first_bbox_to_anchor
            else:
                # 计算后续图例的bbox_to_anchor
                bbox_anchor = (first_bbox_to_anchor[0], first_bbox_to_anchor[1] - i * offset)

        legend = axis.legend(legend_lines, legend_labels, loc=loc, bbox_to_anchor=bbox_anchor, ncol=ncol, frameon=False, **kwargs)
        axis.add_artist(legend)

        
if __name__ == '__main__':
    

    # 示例使用这个函数
    fig, ax = plt.subplots()
    handlers = [ax.scatter(range(10), [i * x for x in range(10)], label=f'Line {i}') for i in range(7)]
    
    # 调用函数，横向排列图例
    custom_legend_layout(ax, n_items=3, offset=0.25, vertical=True,
                         loc='upper center', first_bbox_to_anchor=(0.2, 0.8))
    
    
    from matplotlib.legend import _get_legend_handles_labels as get_legend_handles_labels
    
    handles,labels=get_legend_handles_labels([ax])
    
    plt.show()