Canculating the Compound Annual Growth Rate

I'm trying to calculate the compound annual growth rate of my data (snipet shown below), does anyone know the best way to do this or if there is a function that does part of the job?

Data: (only woried about the preds column here, others can be ignored)

    year month timestep     ymin     ymax          preds       date
1   1998     1        1 17.84037 18.58553      18.21295 1998-01-01
2   1998     2        2 17.05009 17.70642      17.37826 1998-02-01
3   1998     3        3 16.97067 17.61320      17.29193 1998-03-01
4   1998     4        4 18.38551 19.00838      18.69695 1998-04-01
5   1998     5        5 21.39082 21.97338      21.68210 1998-05-01
6   1998     6        6 24.77679 25.35464      25.06571 1998-06-01
7   1998     7        7 27.27057 27.82818      27.54938 1998-07-01
8   1998     8        8 28.24703 28.76702      28.50702 1998-08-01
9   1998     9        9 27.72370 28.24619      27.98494 1998-09-01
10  1998    10       10 25.83783 26.33969      26.08876 1998-10-01
11  1998    11       11 22.94968 23.42268      23.18618 1998-11-01
12  1998    12       12 19.50499 20.05466      19.77982 1998-12-01
13  1999     1       13 17.98323 18.50530      18.24426 1999-01-01
14  1999     2       14 17.20124 17.61746      17.40935 1999-02-01
15  1999     3       15 17.11064 17.53492      17.32278 1999-03-01