如何从使用markevery参数绘制的线中提取标记数据点？

Question

I am trying to get the data coordinates of line markers drawn with a help of markevery option. My attempt at the code below shows that the retrieved (x,y) data contains all original data values and the argument is ignored:

import numpy as np
from matplotlib import pyplot as plt

line, = plt.plot(np.arange(100)**2, marker='o', markevery=(0, 0.1))

x, y = line.get_data()
assert len(x) != 100

Note the value of markevery argument—advanced indexing on the data in this case is not applicable as an alternative.

Of course, I can successfully use private utility method lines._mark_every_path() to solve the problem I deal with. But is there any more, let's say, "correct" way to do that?

Answer 1

The markevery option allows to subsample the marked datapoints on a line. Which points are marked or not is only determined at the moment the line is drawn. There is hence no attribute of the line that would store the data of the shown points, because they might change all the time, depending on the size and limits of the plot.

The solution is indeed to replicate the drawing behaviour of the line to access the coordinates of the marked points, using _mark_every_path .

import numpy as np
from matplotlib import pyplot as plt
from matplotlib import lines

line, = plt.plot(np.arange(100)**2, marker='o', markevery=(0, 0.1))

tpath, affine = line._get_transformed_path().get_transformed_points_and_affine()
p = lines._mark_every_path(line.get_markevery(), 
                      tpath, affine, line.axes.transAxes)

print(len(p.vertices)) # prints 10
print(p.vertices)      # print array of points, shape (10,2)
plt.show()