来自事件坐标的Matplotlib日期时间

Question

I'm sure this is a duplicate of some other question somewhere, but I'm not able to find any answer... so sorry for that, any link would also be apprechiated ;)

I have the x-axis as datetime and would like to get the datetime where a click happened. instead I get some coordinates which I do not understand. How do I convert these coordinates into a datetime (str)?

Consider the following example:

import matplotlib.pyplot as plt
import pandas as pd
import datetime as dt
import mpldatacursor        # pip install mpldatacursor

df = pd.DataFrame(index=[dt.datetime.now() + dt.timedelta(i) for i in range(20)], 
                  data={'val1': [10/(i+1) for i in range(20)],
                        'val2': [5 * 2**(-i) for i in range(20)]})
fig, ax = plt.subplots()
df.plot(ax=ax)

# mpldatacursor opens a dragable yellow rectangle with the informations
# for that point. The kwargs `x` and `y` seem to be the coords.
def myformatter(**kwargs):
    # For one dataset it worked specifying `unit='m'`, coincidence?
    kwargs['x2'] = pd.to_datetime(kwargs['x']).strftime('%Y-%m-%d %H:%M:%S')
    kwargs['x3'] = pd.to_datetime(kwargs['x'], unit='m').strftime('%Y-%m-%d %H:%M:%S')
    kwargs['x4'] = pd.to_datetime(kwargs['x'], unit='h').strftime('%Y-%m-%d %H:%M:%S')
    label = ('t: {x}\nt(ns): {x2}\nt(m): {x3}\nt(h): {x4}\ny: {y:.10}'.format(**kwargs))
    return label
mpldatacursor.datacursor(axes=ax, formatter=myformatter,
                         bbox=dict(fc='yellow', alpha=0.75), draggable=True)

# To compare the coords.
def onclick(event):
   print('data coords %f %f' % (event.xdata, event.ydata))
plt.connect('button_press_event', onclick)

plt.show()

You can click at the plot and it will open a yellow pop-up showing the informations of the selected point. Moreover, the classical way to connect a function to matplotlib prints the current coordinates.

Output: data coords 736772.234764 1.623170

Note: the displayed value t: 736772.230336 is almost the same as event.xdata (736772.234764) in the connected function. I would assume, mpldatacursor takes just the coordinates of the closest datapoint.

How do I convert that value into a string of the form '%Y-%m-%d %H:%M:%S' ?

Answer 1

The units of the matplotlib plot are days since the year one (plus one day). As the documentation tells us:

Matplotlib represents dates using floating point numbers specifying the number of days since 0001-01-01 UTC, plus 1.
For example, 0001-01-01, 06:00 is 1.25, not 0.25. Values < 1, ie dates before 0001-01-01 UTC are not supported.

To convert this number to datetime use matplotlib.dates.num2date() .

d = 736772.230336
matplotlib.dates.num2date(d).strftime('%Y-%m-%d %H:%M:%S')
# '2018-03-19 05:31:41'