i = ['1989-Jan','1989-Feb','1989-Mar','1989-Apr','1989-May']
d = [1,3,7,5,4]

df = pd.DataFrame(index=i, data=d)
df.index = pd.to_datetime(df.index,format='%Y-%b')
df.index = df.index.to_period('M')

result = [0.69, 1.05, 1.41, 1.76, 2.12, 2.57, 3.76, 5.11, 6.46, 7.76, 7.62,
       6.8 , 5.98, 5.17, 4.6 , 4.44, 4.29, 4.15, 4.  , 3.85]

input = [10., 12., 15., 13.,  9., 20., 17., 21., 23., 16., 15., 16.]
index_input = [Period('2000-01', 'M'), Period('2000-02', 'M'),
       Period('2000-03', 'M'), Period('2000-04', 'M'),
       Period('2000-05', 'M'), Period('2000-06', 'M'),
       Period('2000-07', 'M'), Period('2000-08', 'M'),
       Period('2000-09', 'M'), Period('2000-10', 'M'),
       Period('2000-11', 'M'), Period('2000-12', 'M')]

output = [ 9.14,  9.2 ,  9.26,  9.31,  9.37,  9.43,  9.49,  9.54,  9.6 ,
        9.66,  9.71,  9.77,  9.83,  9.89,  9.94, 10.  , 10.06, 10.11,
       10.17, 10.23, 10.29, 10.34, 10.4 , 10.46, 10.51, 10.57, 10.63,
       10.69, 10.74, 10.8 , 10.86, 10.91, 10.97, 11.03, 11.09, 11.14,
       11.2 , 11.26, 11.31, 11.37, 11.43, 11.49, 11.54, 11.6 , 11.66,
       11.72, 11.85, 11.99, 12.12, 12.26, 12.39, 12.53, 12.67, 12.8 ,
       12.94, 13.07, 13.21, 13.34, 13.48, 13.62, 13.75, 13.89, 14.02,
       14.16, 14.3 , 14.43, 14.57, 14.7 , 14.84, 14.97, 15.11, 15.25,
       15.38, 15.52, 15.65, 15.76, 15.72, 15.65, 15.59, 15.52, 15.45,
       15.38, 15.31, 15.25, 15.18, 15.11, 15.04, 14.98, 14.91, 14.84,
       14.77, 14.7 , 14.64, 14.57, 14.5 , 14.43, 14.36, 14.3 , 14.23,
       14.16, 14.09, 14.03, 13.96, 13.89, 13.82, 13.75, 13.59, 13.33,
       13.07, 12.81, 12.55, 12.29, 12.03, 11.77, 11.51, 11.25, 10.99,
       10.73, 10.47, 10.21,  9.95,  9.69,  9.43,  9.17,  8.91,  8.65,
        8.39,  8.13,  7.87,  7.61,  7.35,  7.09,  6.83,  6.58,  6.32,
        6.06,  5.9 ,  6.36,  6.93,  7.5 ,  8.06,  8.63,  9.2 ,  9.77,
       10.33, 10.9 , 11.47, 12.03, 12.6 , 13.17, 13.74, 14.3 , 14.87,
       15.44, 16.  , 16.57, 17.14, 17.7 , 18.27, 18.84, 19.41, 19.97,
       20.54, 21.11, 21.67, 22.24, 22.81, 22.96, 22.71, 22.45, 22.19,
       21.93, 21.68, 21.42, 21.16, 20.9 , 20.65, 20.39, 20.13, 19.87,
       19.62, 19.36, 19.1 , 18.84, 18.59, 18.33, 18.07, 17.81, 17.56,
       17.3 , 17.04, 16.78, 16.53, 16.27, 16.01, 15.75, 15.5 , 15.3 ,
       15.44, 15.63, 15.83, 16.03, 16.22, 16.42, 16.62, 16.82, 17.01,
       17.21, 17.41, 17.6 , 17.8 , 18.  , 18.19, 18.39, 18.59, 18.79,
       18.98, 19.18, 19.38, 19.57, 19.77, 19.97, 20.17, 20.36, 20.56,
       20.76, 20.95, 21.15, 21.34, 21.46, 21.56, 21.67, 21.78, 21.88,
       21.99, 22.1 , 22.21, 22.31, 22.42, 22.53, 22.64, 22.74, 22.85,
       22.96, 23.06, 23.17, 23.28, 23.39, 23.49, 23.6 , 23.71, 23.81,
       23.92, 24.03, 24.14, 24.24, 24.35, 24.46, 24.57, 24.46, 24.13,
       23.81, 23.48, 23.16, 22.83, 22.51, 22.19, 21.86, 21.54, 21.21,
       20.89, 20.56, 20.24, 19.91, 19.59, 19.27, 18.94, 18.62, 18.29,
       17.97, 17.64, 17.32, 16.99, 16.67, 16.34, 16.02, 15.7 , 15.37,
       15.05, 14.76, 14.73, 14.73, 14.74, 14.74, 14.75, 14.75, 14.76,
       14.76, 14.77, 14.77, 14.78, 14.79, 14.79, 14.8 , 14.8 , 14.81,
       14.81, 14.82, 14.82, 14.83, 14.83, 14.84, 14.84, 14.85, 14.85,
       14.86, 14.86, 14.87, 14.87, 14.88, 14.9 , 14.94, 14.97, 15.01,
       15.05, 15.08, 15.12, 15.16, 15.19, 15.23, 15.27, 15.3 , 15.34,
       15.38, 15.41, 15.45, 15.49, 15.52, 15.56, 15.6 , 15.63, 15.67,
       15.71, 15.74, 15.78, 15.82, 15.85, 15.89, 15.93, 15.96, 16.  ,
       16.04, 16.07, 16.11, 16.15, 16.18, 16.22, 16.26, 16.29, 16.33,
       16.37, 16.4 , 16.44, 16.48, 16.51, 16.55]
index_output = pd.date_range(start='2000-01-01', end='2000-12-31', freq='D').to_period('D')