如何在matplotlib中的一个图上绘制由不同日期但相同时间戳组成的时间序列

Question

I have data that shows some values collected on three different dates: 2015-01-08, 2015-01-09 and 2015-01-12. For each date there are several data points that have timestamps.

Date/times are in a list and it looks as follows:

['2015-01-08-09:00:00', '2015-01-08-10:00:00', '2015-01-08-11:00:00', '2015-01-08-12:00:00', '2015-01-08-13:00:00', '2015-01-09-14:00:00', '2015-01-09-15:00:00', '2015-01-09-16:00:00', '2015-01-12-09:00:00', '2015-01-12-10:00:00', '2015-01-12-11:00:00']

On the other hand I have corresponding values (floats) in another list:

[12210.0, 12210.0, 12180.0, 12240.0, 12250.0, 12420.0, 12390.0, 12400.0, 12380.0, 12450.0, 12460.0]

To put all this together and plot a graph I use following code:

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import matplotlib.dates as md
import dateutil
from matplotlib.font_manager import FontProperties

timestamps = ['2015-01-08-09:00:00', '2015-01-08-10:00:00', '2015-01-08-11:00:00', '2015-01-08-12:00:00', '2015-01-08-13:00:00', '2015-01-09-14:00:00', '2015-01-09-15:00:00', '2015-01-09-16:00:00', '2015-01-12-09:00:00', '2015-01-12-10:00:00', '2015-01-12-11:00:00']

ticks = [12210.0, 12210.0, 12180.0, 12240.0, 12250.0, 12420.0, 12390.0, 12400.0, 12380.0, 12450.0, 12460.0]

plt.subplots_adjust(bottom=0.2)
plt.xticks( rotation=90 )

dates = [dateutil.parser.parse(s) for s in timestamps]

ax=plt.gca()
ax.set_xticks(dates)
ax.tick_params(axis='x', labelsize=8)

xfmt = md.DateFormatter('%H:%M:%S')
ax.xaxis.set_major_formatter(xfmt)

plt.plot(dates, ticks, label="Price")

plt.xlabel("Date and time", fontsize=12)
plt.ylabel("Price", fontsize=12)
plt.suptitle("Price during last three days", fontsize=12)

plt.legend(loc=0,prop={'size':8})

plt.savefig("figure.pdf")

When I try to plot these datetimes and values I get a messy graph with the line going back and forth.

It looks like the dates are being ignored and only timestamps are taken in account which is the reason for the messy chart. I tried to edit the datetimes to have the same date and consecutive timestamps and it fixed the chart. However, I must have dates as well..

What am I doing wrong?

Answer 1

When I try to plot these datetimes and values I get a messy graph with the line going back and forth.

Your plots are going all over the place because plt.plot connects the dots in the order you give it. If this order is not monotonically increasing in x, then it looks "messy". You can sort the points by x first to fix this. Here is a minimal example:

import numpy as np
import pylab as plt

X = np.random.random(20)
Y = 2*X+np.random.random(20)

idx = np.argsort(X)
X2 = X[idx]
Y2 = Y[idx]

fig,ax = plt.subplots(2,1)
ax[0].plot(X,Y)
ax[1].plot(X2,Y2)
plt.show()