New tutorial, more advanced this time ! Let’s say we have a number of observations, like occurrences of earthquakes, or visitors connecting to a webserver, etc. These observations don’t occur every second, they are sparse on the time axis. To prepare an example, I’ve created a set of random datetimes like this :

N = 100000 starttime = time.time() basetimes = sorted(np.random.random(N)*np.random.random(N)*1.0e3+starttime) times = [datetime.datetime(*time.gmtime(a)[:7]) for a in basetimes] for i, atime in enumerate(times): times[i] = atime + datetime.timedelta(microseconds=(basetimes[i]-int(basetimes[i])) * 1e6)

N being the number of points we want. We also need to loop over the original times array to add the microseconds, because time.gmtime doesn’t take them into account. This *times* array can be plotted using:

fig = plt.figure() ax = plt.subplot(211) plt.scatter(times,np.random.random(len(times)),alpha=0.1) ax.xaxis_date() plt.grid(True) plt.title('Random datetimes plotted at random y values')

Note we add some random values for the Y axis, just to make a nice plot !

So, now we have an array of datetime.datetime objects and we want to get the number of occurrences on a certain time span, let’s say, 5 seconds. First, to work easily, we convert the array to a numpy array. The binning will be done using the powerful itertools.groupby method. It takes two arguments: an iterable, here our *times* array, and a key, which can be the name of a method.

`itertools.``groupby`(*iterable*[,*key*])- the
*key*in our case will be a method that will return the timestamp int-divided by the time span we have. So, for example, occurences at 1000.02, 1001.12, 1004.66 will get the same integer value of 200, because int(1000.02)/5=200, e.g.

SECOND = 1 MINUTE = SECOND * 60 HOUR = MINUTE * 60 DAY = HOUR * 24 binning = 5*SECOND def group(di): return int(calendar.timegm(di.timetuple()))/binning list_of_dates = np.array(times,dtype=datetime.datetime) grouped_dates = [[datetime.datetime(*time.gmtime(d*binning)[:6]), len(list(g))] for d,g in itertools.groupby(list_of_dates, group)] grouped_dates = zip(*grouped_dates)

- This code first shows the definition of the
*binning*(“time-span”) and the*group*function that will return the*key*for the*groupby*method to group the data. The output of this method are two arrays*d*and*g*, containing the*key*and the elements matching each*key*, respectively. We then, in the long one-liner, create an array of datetime.datetime objects, multiplying back the*key*(=*d*) by the*binning*value to get the left corner of the bin and getting the length of the listed*g*array, the number of occurrences for each*key*. Finally, the grouped_dates array is zipped to get two arrays that will be easy to plot. The rest is super simple:

ax = plt.subplot(212,sharex=ax) plt.bar(grouped_dates[0],grouped_dates[1],width=float(binning)/DAY) ax.xaxis_date() plt.grid(True) plt.title('Number of random datetimes per %i seconds' % binning)

- We just have to create a bar plot of the
*left=keys*,*height=”length of the group*” and*width= “the time span”*(relative to the matplotlib.dates default unit, 1 DAY) and voilà, it’s done! - As always, I’d be happy to hear about your experiments/applications of this tutorial !
- The full code is after the break:

import numpy as np import matplotlib.pyplot as plt import datetime, time, calendar from matplotlib.dates import num2date, DateFormatter import itertools N = 100000 starttime = time.time() basetimes = sorted(np.random.random(N)*np.random.random(N)*1.0e3+starttime) times = [datetime.datetime(*time.gmtime(a)[:7]) for a in basetimes] for i, atime in enumerate(times): times[i] = atime + datetime.timedelta(microseconds=(basetimes[i]-int(basetimes[i])) * 1e6) SECOND = 1 MINUTE = SECOND * 60 HOUR = MINUTE * 60 DAY = HOUR * 24 binning = 5*SECOND def group(di): return int(calendar.timegm(di.timetuple()))/binning list_of_dates = np.array(times,dtype=datetime.datetime) grouped_dates = [[datetime.datetime(*time.gmtime(d*binning)[:6]), len(list(g))] for d,g in itertools.groupby(list_of_dates, group)] grouped_dates = zip(*grouped_dates) #Let's plot ! fig = plt.figure() ax = plt.subplot(211) plt.scatter(times,np.random.random(len(times)),alpha=0.1) ax.xaxis_date() plt.grid(True) plt.title('Random datetimes plotted at random y values') ax = plt.subplot(212,sharex=ax) plt.bar(grouped_dates[0],grouped_dates[1],width=float(binning)/DAY) ax.xaxis_date() plt.grid(True) plt.title('Number of random datetimes per %i seconds' % binning) plt.show()

I have four coloum and the first coloum is data time in 5 minutes interval (e.g [09:00, 09:05,09:10… 15:00]). another coloum is another data as y values. I would like to ask question, how to make first coloum as X axis in 5 minutes interval and another coloum as y values? what library i should learn? Thank you before…