Warming stripes are not warming in the correct place

[khider]

Describe the bug
When running with Kaplan SST, suggests that the warmest period is 1940-1960.

To Reproduce
Data is from: https://github.com/LinkedEarth/PyleoTutorials/blob/main/data/wtc_test_data_nino_even.csv

import pyleoclim as pyleo
import pandas as pd

df = pd.read_csv('../data/wtc_test_data_nino_even.csv')
df.head()

ts_nino = pyleo.Series(time = df['t'], value = df['nino'], label = 'Kaplan Niño3 SST',
time_name = 'Year', value_name = 'NINO3 index',
time_unit = 'CE', value_unit = '$^{\circ}$C')

fig, ax = ts_nino.plot()

fig, ax = ts_nino.stripes(ref_period=(1961,1990), show_xaxis=True)

Expected behavior
Appropriate warm intervals

Screenshots
If applicable, add screenshots to help explain your problem.

[CommonClimate]

addressed by #398