python - How do I set the tick marks for the end of the month in Matplotlib

Question

I have a list of dates that fall on the end of each month, and I would like for the gridlines and labels of my plot to coincide with these dates. Currently I can only get the chart to plot at the beginning of the month. Is there a way to force Matplotlib to use the end of month values in its charting?

import matplotlib.pyplot as plt 
from datetime import datetime
from matplotlib.dates import MonthLocator
from matplotlib.dates import DateFormatter
  
dts = [datetime(2020, 11, 30), datetime(2020, 12, 31),
           datetime(2021, 1, 31), datetime(2021, 2, 28),
           datetime(2021, 3, 31), datetime(2021, 4, 30),
           datetime(2021, 5, 31), datetime(2021, 6, 30),
           datetime(2021, 7, 31), datetime(2021, 8, 31),
           datetime(2021, 9, 30), datetime(2021, 10, 31),
           datetime(2021, 11, 30), datetime(2021, 12, 31),
           datetime(2022, 1, 31), datetime(2022, 2, 28),]
vals = [2000, 2500, 3000, 4000, 4500, 4000, 3000, 2500, 4000, 4500, 3000, 
             2500, 6000, 4000, 3000, 5000]
    
figsize=(3, 6)
fig, ax = plt.subplots(figsize=figsize)
    
ax.xaxis.set_major_locator(MonthLocator((3, 6, 9, 12)))
fmt = DateFormatter("%Y-%m")
ax.xaxis.set_major_formatter(fmt)

ax.plot(dts, vals, color = 'blue')
ax.grid(True)
plt.show()

Answer 1

One option is just to manually or dynamically set_ticks() with:

ax.set_xticks([datetime(2020, 12, 31), datetime(2021, 3, 31), datetime(2021, 6, 30), datetime(2021, 9, 30), datetime(2021, 12, 31)])

OR Better:

ax.set_xticks(dts[1::3]) # this uses every third element starting with second element

Full code:

import matplotlib.pyplot as plt 
from datetime import datetime
from matplotlib.dates import MonthLocator
from matplotlib.dates import DateFormatter

dts = [datetime(2020, 11, 30), datetime(2020, 12, 31),
           datetime(2021, 1, 31), datetime(2021, 2, 28),
           datetime(2021, 3, 31), datetime(2021, 4, 30),
           datetime(2021, 5, 31), datetime(2021, 6, 30),
           datetime(2021, 7, 31), datetime(2021, 8, 31),
           datetime(2021, 9, 30), datetime(2021, 10, 31),
           datetime(2021, 11, 30), datetime(2021, 12, 31),
           datetime(2022, 1, 31), datetime(2022, 2, 28),]
vals = [2000, 2500, 3000, 4000, 4500, 4000, 3000, 2500, 4000, 4500, 3000, 
             2500, 6000, 4000, 3000, 5000]

figsize=(12, 6)
fig, ax = plt.subplots(figsize=figsize)

fmt = DateFormatter("%Y-%m-%d")
ax.xaxis.set_major_formatter(fmt)
ax.set_xticks([datetime(2020, 12, 31), datetime(2021, 3, 31), datetime(2021, 6, 30), datetime(2021, 9, 30), datetime(2021, 12, 31)])
ax.set_xticks(dts[1::3])

ax.plot(dts, vals, color = 'blue')
ax.grid(True)
plt.show()