在X轴上调整时间戳-Matplotlib

Question

I am trying to create a line plot in order of time . For the df below, the first value appears at 07:00:00 and finishes at 00:00:40 .

But the timestamps aren't assigned to the x-axis and the row after midnight is plotted first, instead of last.

import pandas as pd
import matplotlib.pyplot as plt

d = ({
    'Time' : ['7:00:00','10:30:00','12:40:00','16:25:00','18:30:00','22:40:00','00:40:00'],
    'Value' : [1,2,3,4,5,4,10],           
     })

df = pd.DataFrame(d)

df['Time'] = pd.to_timedelta(df['Time'])

plt.plot(df['Time'], df['Value'])

plt.show()

print(df)

Answer 1

Your timedelta object is being converted to a numerical representation by matplotlib. That's why you aren't getting a date on your x axis. And the plot is going in order. It's just that '00:40:00' is less than all the other times so it's being plotted as the left most point.

What you can do instead is use a datetime format to include days, which will indicate that 00:40:00 should come last since it'll fall on the next day. You can also use pandas plotting method for easier formatting:

d = ({
    'Time' : ['2019/1/1 7:00:00','2019/1/1 10:30:00','2019/1/1 12:40:00',
              '2019/1/1 16:25:00','2019/1/1 18:30:00','2019/1/1 22:40:00',
              '2019/1/2 00:40:00'],
    'Value' : [1,2,3,4,5,4,10],           
})

df = pd.DataFrame(d)
df['Time'] = pd.to_datetime(df['Time'])

df.plot(x='Time', y='Value')

---

Update

To set the tick/tick lables at your time points is a bit tricky. This post will give you an idea of how the positioning works. Basically, you'll need to use something like matplotlib.dates.date2num to get the numerical representation of datetime:

xticks = [matplotlib.dates.date2num(x) for x in df['Time']]
xticklabels = [x.strftime('%H:%M') for x in df['Time']]

ax.set_xticks(xticks)
ax.set_xticklabels(xticklabels)