Matplotlib and timestamp are causing troubles

Question

Why is this not working? How to fix it? I am trying to plot a figure where x-axis is the temporal axis and y values are the second column in csv file. I want the x plot labels to be in a more human readable format...

testi.csv

1315655275.0,0.1,1.4
1325655275.0,0.11,2.2
1335655275.0,0.23,11.96
1345655275.0,0.81,63.18
1355655275.0,2.76,560.28
1365655275.0,5.54,609.4
1375655275.0,7.21,576.8

testi2.py

import numpy as np
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import matplotlib.dates as md
data = np.genfromtxt('testi.csv', delimiter=',', names=['t', 'btc', 'eur'])
plt.xkcd()
plt.plot(data['t'], data['btc'], label='the data')
plt.xticks(rotation=75)
ax=plt.gca()
xfmt = md.DateFormatter('%Y-%m-%d %H:%M:%S')
ax.xaxis.set_major_formatter(xfmt)
plt.savefig('testi2.png')

This is the error message after I give "python < testi2.py":

Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/pyplot.py", line 564, in savefig
    return fig.savefig(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/figure.py", line 1421, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/backend_bases.py", line 2220, in print_figure
    **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/backends/backend_agg.py", line 505, in print_png
    FigureCanvasAgg.draw(self)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/backends/backend_agg.py", line 451, in draw
    self.figure.draw(self.renderer)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/figure.py", line 1034, in draw
    func(*args)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/axes/_base.py", line 2056, in draw
    a.draw(renderer)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/axis.py", line 1087, in draw
    ticks_to_draw = self._update_ticks(renderer)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/axis.py", line 945, in _update_ticks
    tick_tups = [t for t in self.iter_ticks()]
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/axis.py", line 893, in iter_ticks
    for i, val in enumerate(majorLocs)]
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/dates.py", line 395, in __call__
    dt = num2date(x, self.tz)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/dates.py", line 329, in num2date
    return _from_ordinalf(x, tz)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib/dates.py", line 218, in _from_ordinalf
    dt = datetime.datetime.fromordinal(ix)
ValueError: year is out of range

Answer 1

You have to convert your timestamps to datetime objects.

import numpy as np
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import matplotlib.dates as md
import datetime as dt

# create vectorized converter (can take list-like objects as arguments)
dateconv = np.vectorize(dt.datetime.fromtimestamp)

data = np.genfromtxt('testi.csv', delimiter=',', names=['t', 'btc', 'eur'])
plt.xkcd()

dates = dateconv(data['t']) # convert timestamps to datetime objects

plt.plot(dates, data['btc'], label='the data')
plt.xticks(dates, rotation=75, ha='right') # set ticks at plotted datetimes
ax=plt.gca()
xfmt = md.DateFormatter('%Y-%m-%d %H:%M:%S')
ax.xaxis.set_major_formatter(xfmt)

plt.tight_layout() # to make sure everything fits inside the figures boundaries

plt.savefig('testi2.png')

In order to make the label you set for the plot appear, you have to call plt.legend() before plt.savefig('testi2.png'). With plt.legend(loc=2) (places the legend in upper left corner) the output becomes: