Formatting of Dates in Python with matplotlib and datetime: x axis error - float

Question

I try to plot a graph with a set of data. The y-axis is no problem, it's simple floats. Problem is the x-axis: Its data is formatted by datetime to '%Y-%m-%d %H:%M:%S'. When trying to plot it occurs of course the error of no float possible... I tried quite many ways and it still wouldn't work...

So input so far are two arrays:

x is ['2016-02-05 17:14:55', '2016-02-05 17:14:51', '2016-02-05 17:14:49', ...].
y is ['35.764299', '20.3008', '36.94704', ...]

Answer 1

You can make use mapplotlib's DateFormatter:

1. Parse your date strings into datetime objects.
2. Convert your datetime objects into matplotlib numbers using date2num()
3. Create a DateFormatter() using your desired output format
4. Apply the DataFormatter() as the major tick format for the x-axis.
5. Also convert your float strings to actual floats.

Try the following:

import matplotlib
import matplotlib.pyplot as plt
from datetime import datetime

x_orig = ['2016-02-05 17:14:55', '2016-02-05 17:14:51', '2016-02-05 17:14:49']
x = [datetime.strptime(d, '%Y-%m-%d %H:%M:%S') for d in x_orig]
y = ['35.764299', '20.3008', '36.94704']

xs = matplotlib.dates.date2num(x)
y_float = list(map(float, y))      # Convert y values to floats
hfmt = matplotlib.dates.DateFormatter('%H:%M:%S')

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
ax.xaxis.set_major_formatter(hfmt)
plt.setp(ax.get_xticklabels(), rotation=15)
ax.plot(xs, y_float)
plt.show()

This would display the following: