Python: DateTime-Objects can be plotted in matplotlib, but only sometimes?

Question

So, I made a DataFrame that looks like this:

The DF ist chronologically ordered by the DateTime-Objects. These DateTimes are generated by transforming the column "attributes.timestamp" which contains timestamps as strings:

df["DateTime"] = df["attributes.timestamp"].apply(lambda x: datetime.strptime(x, '%Y-%m-%dT%H:%M:%SZ'))

The corresponding y-values is a counter that counts objects within the corresponding minute. When I try to plot this DF in matplotlib, it actually works. It takes the datetime-objects as x-values and plots the counts for that minute as follows:

Now of course it looks dumb to have to full DateTime-object shown on the x-axis. It shows month, day and hour in this order (in the example it's the 2nd of March from 2pm to 20pm). I want it to show JUST the hours (or at least just the time, not the entire date that comes with it). So I tried to add a new column (called "Time") to the DF. That column would extract the time from the DateTime column using the following code:

df["Time"] = df["DateTime"].time()

However, that doesn't work, for it gives me the attribute error "'Series' object has no attribute 'time'". Instead I tried something else. I just repeated the whole code I used earlier when I generated the DateTime objects and added ".time()" to it.

df["Time"] = df["attributes.timestamp"].apply(lambda x: datetime.strptime(x, '%Y-%m-%dT%H:%M:%SZ').time())

I have no idea why, but now it works just fine. I was capable of adding the time from my Datetime-object:

My next idea would be to use the "Time" column on my x-axis instead of the whole datetime for plotting. y-values from the counter stay the same. But for some reason that doesn't work. When I try to plot it like that, it gives me the following error: TypeError: float() argument must be a string or a number, not 'datetime.time' Strangely enough that was no problem when plotting with the whole DateTime-object. I don't know, why the exctracted time would be a problem, since it is a chronologically ordered value as well.

My question is: Why the heck does my approach not work? And: Is there any way to go around this?

that does indeed also extract the time. When plotting it on the x-axis it still gives me the same error though: TypeError: float() argument must be a string or a number, not 'datetime.time' — Rafa
– Rafa, Commented May 9, 2020 at 23:28

tnwei · Accepted Answer · 2020-05-10 05:34:07Z

1

Matplotlib supports plotting pandas DatetimeIndex, as well as numpy datetime64 objects, but not sequences of datetime.time. In addition, df["Time"] = df["DateTime"].time() does not work because you are applying the .time() method to the Series itself, instead of to the elements of the Series within, which are pandas.Timestamp objects that do have the .time() method defined.

To answer your main question, you just want the x-axis to not show redundant info, yes? Instead of creating a new column only for itme, the proper way to do this is to format the matplotlib x-axis with matplotlib.dates.DateFormatter.

Here's a minimal example:

import matplotlib.pyplot as plt
import pandas as pd

# Example DatetimeIndex and data
x = pd.date_range(start='2020-05-10', end='2020-05-11', freq='1h')
y = list(range(len(x)))

fig, ax = plt.subplots()
plt.plot(x, y)

# The following specifies the format for dates
import matplotlib.dates as mdates
date_fmt = mdates.DateFormatter('%I: %M%p')
ax.xaxis.set_major_formatter(date_fmt)

# autofmt_xdate helps with auto-rotating dates so they do not overlap
fig.autofmt_xdate()
plt.show()

As for how to know what string to pass to DateFormatter, refer to https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior for strftime formats.

Matplotlib has a page dedicated to fixing common date annoyances that you might find useful: https://matplotlib.org/3.1.1/gallery/recipes/common_date_problems.html

answered May 10, 2020 at 5:34

tnwei

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1 Comment

Rafa Over a year ago

Thanks mate, you really helped me out big time! Would give you a thumbs up, if I could. But it wont let me :P

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