Line3D

You can get get the original data from the (private) _verts3d attribute

xdata, ydata, zdata = line._verts3d
print(xdata)

Complete example

import matplotlib as mpl
from mpl_toolkits.mplot3d import Axes3D
import numpy as np
import matplotlib.pyplot as plt

mpl.rcParams['legend.fontsize'] = 10

fig = plt.figure()
ax = fig.gca(projection='3d')

# Prepare arrays x, y, z
x = np.arange(5)
y = np.arange(5)*4
z = np.arange(5)*100

line, = ax.plot(x, y, z, label='curve')

fig.canvas.draw()

xdata, ydata, zdata = line._verts3d
print(xdata)  # This prints [0 1 2 3 4]

plt.show()

Some explanation: The problem with get_data or get_xdata is that it will return the projected coordinates once the figure is drawn. So while before drawing the figure, line.get_xdata() would indeed return the correct values, after drawing, it would return something like

[ -6.14413090e-02  -3.08824862e-02  -3.33066907e-17   3.12113190e-02 6.27567511e-02]

in the above example, which is the x component of the 3D coordinates projected onto 2D.

There is a pull request to matplotlib, which would allow to get the data via methods get_data_3d. This is still not merged, but might allow the above to be done without using private arguments in a future version of matplotlib.

Poly3DCollection

For a plot_surface plot this looks similar, except that the attribute to look at is the ._vec

surf = ax.plot_surface(X, Y, Z)
xdata, ydata, zdata, _ = surf._vec
print(xdata)