Matplotlib：如何单独标记点？

Question

在使用matplotlib这个库时，我用scatter方法画了一些点（见下面的代码）。我想给每个点单独加上标签。

这段代码会用labels数组给每个点加标签，但我希望第一个点用labels[0]来标记，第二个点用labels[1]，以此类推。

import numpy as np; import matplotlib.pyplot as plt
y = np.arange(10) # points to plot
labels = np.arange(10) # labels of the points
fig, ax = plt.subplots(nrows=1, ncols=1)
ax.scatter(x=np.arange(10), y=y, label=labels, picker=3)

有没有什么办法可以做到这一点？顺便问一下，有没有办法遍历ax中的点？ax.get_children()这个方法返回的数据我看不懂。

谢谢！

Answer 1

所有这些内容都可以放在一个函数或类里面，隐藏起来。

# import stuff
import matplotlib.pyplot as plt
import numpy as np

# create dictionary we will close over (twice)
label_dict = dict()
# helper function to do the scatter plot + shove data into label_dict
def lab_scatter(ax, x, y, label_list, *args, **kwargs):
    if 'picker' not in kwargs:
        kwargs['picker'] = 3
    sc = ax.scatter(x, y, *args, **kwargs)
    label_dict[sc] = label_list
    return sc
# call back function which also closes over label_dict, should add more sanity checks
# (that artist is actually in the dict, deal with multiple hits in ind ect)
def cb_fun(event):
    # grab list of labels from the dict, print the right one
    print label_dict[event.artist][event.ind[0]]
# create the figure and axes to use
fig, ax = plt.subplots(1, 1)
# loop over 5 synthetic data sets
for j in range(5):
    # use our helper function to do the plotting
    lab_scatter(ax,
                np.ones(10) * j,
                np.random.rand(10),
                # give each point a unique label
                label_list = ['label_{s}_{f}'.format(s=j, f=k) for k in range(10)])
# connect up the call back function
cid = fig.canvas.mpl_connect('pick_event', cb_fun)

Answer 2

假设你并不是要绘制很多散点图，你可以为每个点单独调用一次 scatter 方法：

import numpy as np; import matplotlib.pyplot as plt
y = np.arange(10) # points to plot
x=np.arange(10)
labels = np.arange(10) # labels of the points
fig, ax = plt.subplots(nrows=1, ncols=1)
for x_,y_,label in zip(x,y,labels):
    ax.scatter([x_], [y_], label=label, picker=3)

如果你要绘制的点数达到几千或几万，那就会开始变得卡顿，不过如果只是几个点，那就没问题。

关于你问题的第二部分，ax.get_children() 会返回一个列表，这个列表包含了构成这些坐标轴的所有对象，比如：

[<matplotlib.axis.XAxis at 0x103acc410>,
 <matplotlib.axis.YAxis at 0x103acddd0>,
 <matplotlib.collections.PathCollection at 0x10308ba10>, #<--- this is a set of scatter points
 <matplotlib.text.Text at 0x103082d50>,
 <matplotlib.patches.Rectangle at 0x103082dd0>,
 <matplotlib.spines.Spine at 0x103acc2d0>,
 <matplotlib.spines.Spine at 0x103ac9f90>,
 <matplotlib.spines.Spine at 0x103acc150>,
 <matplotlib.spines.Spine at 0x103ac9dd0>]

如果你只是想获取坐标轴中的散点集合，最简单的方法是通过 ax.collections。这个 list 包含了在坐标轴中绘制的所有 collections 实例（散点属于 PathCollection）。

In [9]: ax.collections
Out[9]: [<matplotlib.collections.PathCollection at 0x10308ba10>]

如果你为每个点都单独绘制了一个 scatter，那么遍历这些点就非常简单了：

# iterate over points and turn them all red
for point in ax.collections:
    point.set_facecolor("red")