Spline

R - Code

~/code/R/

Initial Dataset

ger <- data.frame(DATE= as.Date(c("1991-01-01", "1991-04-01", "1991-07-01", "1991-10-01", "1992-01-01" )),
                  VALUE= c(470780, 468834, 466332, 472949, 480359))

ger %>%
  print_table_conditional()

DATE	VALUE
1991-01-01	470780
1991-04-01	468834
1991-07-01	466332
1991-10-01	472949
1992-01-01	480359

Extrapolate

DateSeq <- seq(ger$DATE[1],tail(ger$DATE,1),by="1 month")
gerMonthly <- data.frame(DATE=DateSeq, Interp.Value=spline(ger, method="natural", xout=DateSeq)$y)
merge(ger, gerMonthly, by='DATE', all.y = T) %>%
  print_table_conditional()