z chart

import numpy as np
import pandas as pd


np.random.seed(0)
sales = pd.DataFrame({
    "sale" :np.random.randint(10, high=15, size=24),
}, index=pd.date_range("2015-01-01", "2016-12-01", freq="MS"))

sales["cumsum_12month"] = sales["sale"].rolling(window=12).sum()
sales["2016_cumsum"] = sales["2016-01-01":]["sale"].cumsum()
sales.plot()
sales

z chart

sale cumsum_12month 2016_cumsum
2015-01-01 14 NaN NaN
2015-02-01 10 NaN NaN
2015-03-01 13 NaN NaN
2015-04-01 13 NaN NaN
2015-05-01 13 NaN NaN
2015-06-01 11 NaN NaN
2015-07-01 13 NaN NaN
2015-08-01 12 NaN NaN
2015-09-01 14 NaN NaN
2015-10-01 10 NaN NaN
2015-11-01 10 NaN NaN
2015-12-01 14 147.0 NaN
2016-01-01 12 145.0 12.0
2016-02-01 11 146.0 23.0
2016-03-01 10 143.0 33.0
2016-04-01 11 141.0 44.0
2016-05-01 11 139.0 55.0
2016-06-01 10 138.0 65.0
2016-07-01 11 136.0 76.0
2016-08-01 14 138.0 90.0
2016-09-01 13 137.0 103.0
2016-10-01 10 137.0 113.0
2016-11-01 13 140.0 126.0
2016-12-01 10 136.0 136.0