Importing HH:MM:SS time values results in "Invalid format string" when using plot_date

Question

I have been trying to make a simple plot with an x-axis in the format HH:MM:SS with the values taken from a file in the same format. I'm using numpy.loadtxt and a converter to read the times and values as follows:

time, moisture = np.loadtxt(dirlist[0], usecols=(1,2), unpack=True, converters = {1: dates.strpdate2num("%H:%M:%S")})

However, when I plot using date_plot, I receive the following error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python27\lib\lib-tk\Tkinter.py", line 1486, in __call__
    return self.func(*args)
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 276, in resize
    self.show()
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 348, in draw
    FigureCanvasAgg.draw(self)
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_agg.py", line 451, in draw
    self.figure.draw(self.renderer)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\figure.py", line 1034, in draw
    func(*args)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 2086, in draw
    a.draw(renderer)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\axis.py", line 1091, in draw
    ticks_to_draw = self._update_ticks(renderer)
  File "C:\Python27\lib\site-packages\matplotlib\axis.py", line 945, in _update_ticks
    tick_tups = [t for t in self.iter_ticks()]
  File "C:\Python27\lib\site-packages\matplotlib\axis.py", line 893, in iter_ticks
    for i, val in enumerate(majorLocs)]
  File "C:\Python27\lib\site-packages\matplotlib\dates.py", line 524, in __call__
    return self._formatter(x, pos)
  File "C:\Python27\lib\site-packages\matplotlib\dates.py", line 375, in __call__
    return self.strftime(dt, self.fmt)
  File "C:\Python27\lib\site-packages\matplotlib\dates.py", line 413, in strftime
    s1 = time.strftime(fmt, (year,) + timetuple[1:])
ValueError: Invalid format string

As it looks to me that its complaining about a missing year, I decided to check the parsed date using num2date and the correct date of 1900-01-01 11:10:41+00:00 was returned although I don't know why there is an +00:00. Could this be the source of my woes?

Even though strpdate2num is parsing correctly, I decided to check if I add a YYYY:MM:DD to the beginning of the times within the file would it work and low and behold it did. Unfortunately, I'd rather not append a YYYY:MM:DD to the beginning of every time value so, finally my question is: how do I get plot_date to work while reading in a time without an YYYY:MM:DD prefix and why is strpdate2num not taking care of this?

Cheers, Dave

As requested, a snippet of the input file:

0.465752314814815   11:10:41    9.086096129
0.466226851851852   11:11:22    8.346124754
0.466701388888889   11:12:03    8.179795013
0.467175925925926   11:12:44    8.364891272
0.467662037037037   11:13:26    8.147598262
0.468136574074074   11:14:07    8.35810013
0.468611111111111   11:14:48    8.333816653
0.469085648148148   11:15:29    8.41123069
0.469085648148148   11:15:29    8.130661712
0.469560185185185   11:16:10    3.131862435

Here is my code:

datefunc = lambda x: dates.date2num(datetime.datetime.strptime(x, '%H:%M:%S'))

times, moisture = np.loadtxt(dirlist[0], delimiter='\t', usecols=(1,2), converters={1: datefunc}, unpack=True) 

times_date=[]

for t in times:
    times_date.append(dates.num2date(t)) #Converting back to a datetime object

plt.plot_date(times_date , moisture, xdate=True)
# plt.gcf().autofmt_xdate()
plt.show()
plt.close()

Answer 1

It seems like a strange problem as you're original code looks ok. This works for me in Python 2 (Python 3 requires conversion from byte data). I have tried this with your example data:

import matplotlib.pyplot as plt
from matplotlib.dates import strpdate2num
import numpy as np

time, val = np.loadtxt("File.txt", usecols=(1,2), unpack=True, converters = {1: strpdate2num("%H:%M:%S")})

plt.plot_date(time, val)
plt.show()