an error in python plot

Question

import numpy as np
import matplotlib.pyplot as plt

from mpl_toolkits.mplot3d import Axes3D
from sklearn.linear_model import LinearRegression

a = np.zeros((1000,3))
fig=plt.figure()
ax=fig.gca(projection='3d')
line1,=ax.plot(a[:,0], a[:,1], a[:,2], 'k')
ax.set_xlabel('$x_1_t$');ax.set_ylabel('$x_1_t-tau$');ax.set_zlabel('$x_1_t-2tau$')
plt.title('hello')
plt.show()

I use python2.x. The error message is the following. Anyone knows how to fix it?

Traceback (most recent call last):
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/backends/backend_macosx.py", line 83, in _draw
    self.figure.draw(renderer)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/figure.py", line 1475, in draw
    renderer, self, artists, self.suppressComposite)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/image.py", line 141, in _draw_list_compositing_images
    a.draw(renderer)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mpl_toolkits/mplot3d/axes3d.py", line 298, in draw
    ax.draw(renderer)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mpl_toolkits/mplot3d/axis3d.py", line 306, in draw
    self.label.draw(renderer)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/text.py", line 706, in draw
    bbox, info, descent = textobj._get_layout(renderer)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/text.py", line 309, in _get_layout
    ismath=ismath)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/backends/backend_agg.py", line 237, in get_text_width_height_descent
    self.mathtext_parser.parse(s, self.dpi, prop)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/mathtext.py", line 3293, in parse
    box = self._parser.parse(s, font_output, fontsize, dpi)
   File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/mathtext.py", line 2521, in parse
    six.text_type(err)]))
 ValueError: 
^ Double subscript (at char 0), (line:1, col:1)

Answer 1

The problem is in your labels, you are using a double subscripts:

$x_1_t$

If you change to just using single subscripts it should work (e.g):

$x_1t$

Answer 2

As the other answer suggests, the problem is trying to use double subscripts in the axis labels. The solution depends on how you would like the labels to look.

If you would like the 1 and the t to both be subscripts then you can use:

$x_{1t}$

If you want the first underscore to be an actual underscore then you can use

$x\_1_t$