[BUG] plot_series inconsistent date/time index

Describe the bug

To Reproduce

y = load_airline()
y = y[:48]
fh=np.arange(1, 13)

y_train, y_test = temporal_train_test_split(y, test_size=len(fh))
plot_series(y_train, y_test, labels=["y_train", "y_test"]);
print(y.shape, y_train.shape[0], y_test.shape[0])
print(y.index)

(48,) 36 12 PeriodIndex([‘1949-01’, ‘1949-02’, ‘1949-03’, ‘1949-04’, ‘1949-05’, ‘1949-06’, ‘1949-07’, ‘1949-08’, ‘1949-09’, ‘1949-10’, ‘1949-11’, ‘1949-12’, ‘1950-01’, ‘1950-02’, ‘1950-03’, ‘1950-04’, ‘1950-05’, ‘1950-06’, ‘1950-07’, ‘1950-08’, ‘1950-09’, ‘1950-10’, ‘1950-11’, ‘1950-12’, ‘1951-01’, ‘1951-02’, ‘1951-03’, ‘1951-04’, ‘1951-05’, ‘1951-06’, ‘1951-07’, ‘1951-08’, ‘1951-09’, ‘1951-10’, ‘1951-11’, ‘1951-12’, ‘1952-01’, ‘1952-02’, ‘1952-03’, ‘1952-04’, ‘1952-05’, ‘1952-06’, ‘1952-07’, ‘1952-08’, ‘1952-09’, ‘1952-10’, ‘1952-11’, ‘1952-12’], dtype=‘period[M]’, name=‘Period’, freq=‘M’)

Expected behavior The X-axis should go till end of 1952. The X-axis on the plot does not reflect the indices correctly. It should go till 1952, but shows values only till 1949

Versions System: python: 3.6.12 |Anaconda, Inc.| (default, Sep 9 2020, 00:29:25) [MSC v.1916 64 bit (AMD64)] executable: C:\Users\xxxx\AppData\Local\Continuum\anaconda3\envs\sktime\python.exe machine: Windows-10-10.0.18362-SP0

Python dependencies: pip: 20.3 setuptools: 49.6.0 sklearn: 0.23.2 numpy: 1.19.2 scipy: 1.5.2 Cython: 0.29.17 pandas: 1.1.3 matplotlib: 3.3.2 joblib: 0.17.0 numba: None pmdarima: 1.7.1 tsfresh: None

[mloning]

If you’re interested in looking into this, I’d appreciate a PR!

[gracewgao]

I can try to fix this!