How to receive ticks every hour?

Question

Consider this simple example.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.ticker import FuncFormatter
import matplotlib.dates as mdates

pd.__version__
Out[147]: u'0.22.0'

idx = pd.date_range('2017-01-01 05:03', '2017-01-01 18:03', freq = 'min')

df = pd.Series(np.random.randn(len(idx)),  index = idx)
df.head()
Out[145]: 
2017-01-01 05:03:00   0.4361
2017-01-01 05:04:00   0.9737
2017-01-01 05:05:00   0.8430
2017-01-01 05:06:00   0.4292
2017-01-01 05:07:00   0.5739
Freq: T, dtype: float64

I want to capture this, and there are tics every day. I use:

fig, ax = plt.subplots()
hours = mdates.HourLocator(interval = 1)  #
h_fmt = mdates.DateFormatter('%H:%M:%S')

df.plot(ax = ax, color = 'black', linewidth = 0.4)

ax.xaxis.set_major_locator(hours)
ax.xaxis.set_major_formatter(h_fmt)

which gives

why don't ticks appear every hour here? Thank you for your help!

Answer 1

The problem is that while pandas generally directly wraps matplotlib construction methods, this does not apply to date charts. Once the dates are involved, pandas uses a completely different numeric representation of the dates and therefore also uses its own locators for ticks.

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. df.plot(ax = ax, color = 'black', linewidth = 0.4)

ax.plot(df.index, df.values, color = 'black', linewidth = 0.4)

:

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.dates as mdates

idx = pd.date_range('2017-01-01 05:03', '2017-01-01 18:03', freq = 'min')
df = pd.Series(np.random.randn(len(idx)),  index = idx)

fig, ax = plt.subplots()
hours = mdates.HourLocator(interval = 1)
h_fmt = mdates.DateFormatter('%H:%M:%S')

ax.plot(df.index, df.values, color = 'black', linewidth = 0.4)
ax.xaxis.set_major_locator(hours)
ax.xaxis.set_major_formatter(h_fmt)

fig.autofmt_xdate()
plt.show()

---

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import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.dates as mdates

idx = pd.date_range('2017-01-01 05:03', '2017-01-01 18:03', freq = 'min')

df = pd.DataFrame(np.cumsum(np.random.randn(len(idx), 2),0), 
                  index = idx, columns=list("AB"))

fig, ax = plt.subplots()
ax.plot(df.index, df["A"], color = 'black')
ax2 = ax.twinx()
ax2.plot(df.index, df["B"], color = 'indigo')

hours = mdates.HourLocator(interval = 1)
h_fmt = mdates.DateFormatter('%H:%M:%S')
ax.xaxis.set_major_locator(hours)
ax.xaxis.set_major_formatter(h_fmt)

fig.autofmt_xdate()
plt.show()