Changing Date Format X-Axis Matplotlib - date

Question

I have a plot that I need to change the date format along the x-axis to 'YYYY-mm' format. The plot looks like this:
The code looks like this -
import matplotlib.dates as mdates
import datetime
from matplotlib.dates import DateFormatter
def mean_absolute_percentage_error(y_true, y_pred):
y_true, y_pred = np.array(y_true), np.array(y_pred)
return np.mean(np.abs((y_true - y_pred) / y_true)) * 100
mape = mean_absolute_percentage_error(testarray.monthly_flow, testarray.predicted).round(2) # PRINT MAPE
print(mape)
fpm = (1+mape/100) #'fitted plus mape -- fpm
fmm = (1-mape/100) #'fitted minus mape -- fmm
years = mdates.YearLocator() # every year
months = mdates.MonthLocator() # every month
plus_mape = fitted_series2.multiply(other=fpm)
minus_mape= fitted_series2.multiply(other=fmm)
# Plot WITH MAPE +/-
fig,ax = plt.subplots()
ax.plot(df2.monthly_flow[-24:])
ax.plot(fitted_series2[:12], color='darkgreen')
ax.plot(plus_mape[:12], color='black',linestyle='dotted')
ax.plot(minus_mape[:12], color='black',linestyle='dotted')
ax.fill_between(lower_series.index[:12],
lower_series[:12],
upper_series[:12],
color='k',
alpha=.15)
plt.title("SARIMAX Forecast of Monthly Col River Flow")
plt.show()
date_form = DateFormatter('%YYYY-%mm')
ax.xaxis.set_major_formatter(date_form)
plt.show()
And, despite using the 'major_formatter' with the date format specified, it does nothing as you can see with the above plot. I'm not sure what else to do. Thank you for suggestions,