Plotting a secondary MJD date axis with matplotlib ≥ 3.1

plot_mjd_secondary.py

import matplotlib.pyplot as plt
import numpy as np
from astropy.time import Time
import astropy.units as u


t = Time.now() + np.linspace(0, 30, 30) * u.day

# plot something vs time normally using a list of datetimes as x values
fig, ax = plt.subplots(constrained_layout=True)
ax.plot(t.to_datetime(), np.random.normal(80, 10, 30))

for l in ax.get_xticklabels():
    l.set_ha('right')
    l.set_rotation(30)

# create the secondary axis showing mjd at the top
def plot2mjd(t):
    '''Convert from matplotlib plot date to mjd'''
    return Time(t, format="plot_date").mjd


def mjd2plot(mjd):
    '''Convert from mjd to matplotlib plot'''
    return Time(mjd, format="mjd").plot_date

mjd_ax = ax.secondary_xaxis('top', functions=(plot2mjd, mjd2plot))
mjd_ax.set_xlabel('MJD')


plt.show()

Result: