Creating graph with date and time in axis labels with matplotlib


Question

I have my data in an array of the following structure,

[[1293606162197, 0, 0],
 [1293605477994, 63, 0],
 [1293605478057, 0, 0],
 [1293605478072, 2735, 1249],
 [1293606162213, 0, 0],
 [1293606162229, 0, 0]]

The first column is epoch time (in ms), second is y1 and third is y2. I need a plot with the time on the x-axis, and y1 and y2 on left and right y-axes.

I have been scouring through the documentation but couldn't find any way to get my x-axis ticks to display both date and time, like "28/12 16:48", i.e., "date/month hour:min". All the documentation helps me with is to display dates alone, but that is not what I want. Any help would be appreciated on this.

And, if it may seem like, this is not homework. It is actually a follow up to my previous question, Reading and graphing data read from huge files.

1
30
5/23/2017 12:18:01 PM

Accepted Answer

I hope this helps. I've always had a hard time with matplotlib's dates. Matplotlib requires a float format which is days since epoch. The helper functions num2date and date2num along with python builtin datetime can be used to convert to/from. The formatting business was lifted from this example. You can change an axis on any plot to a date axis using set_major_formatter.

import numpy as np
from matplotlib import pyplot as plt
from matplotlib import dates
import datetime

a = np.array([
    [1293605162197, 0, 0],
    [1293605477994, 63, 0],
    [1293605478057, 0, 0],
    [1293605478072, 2735, 1249],
    [1293606162213, 0, 0],
    [1293606162229, 0, 0]])

d = a[:,0]
y1 = a[:,1]
y2 = a[:,2]

# convert epoch to matplotlib float format
s = d/1000
ms = d-1000*s  # not needed?
dts = map(datetime.datetime.fromtimestamp, s)
fds = dates.date2num(dts) # converted

# matplotlib date format object
hfmt = dates.DateFormatter('%m/%d %H:%M')

fig = plt.figure()
ax = fig.add_subplot(111)
ax.vlines(fds, y2, y1)

ax.xaxis.set_major_locator(dates.MinuteLocator())
ax.xaxis.set_major_formatter(hfmt)
ax.set_ylim(bottom = 0)
plt.xticks(rotation='vertical')
plt.subplots_adjust(bottom=.3)
plt.show()

result

44
4/2/2011 1:12:05 PM

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