$ groupby --help
Usage: groupby [OPTIONS]
Read lines from stdin and group lines by key.

  -w, --width=N              Use N chars as key[0].
  -f, --fields=N             Use N fields as key[1].
  -s, --skip=N               Skip N chars/fields[0].
  -d, --delimiters=DELIM     Use DELIM as field delimiter["\t "].
  -j, --join-delimiter=DELIM Join lines by DELIM["\t"].
      --help                 Display this help and exit.

$ cat input
1,2013,1,3000
2,2013,2,8800
3,2013,3,9500
4,2013,4,5700
5,2013,5,2600
6,2013,6,4100
7,2013,7,9900
8,2013,8,900
9,2013,9,7100
10,2013,10,8300
11,2013,11,7700
12,2013,12,1200
13,2014,1,8300
14,2014,2,3200
15,2014,3,7100
16,2014,4,3900
17,2014,5,4300
18,2014,6,7700
19,2014,7,9000
20,2014,8,2900
21,2014,9,8300
22,2014,10,6500
23,2014,11,1700
24,2014,12,3700

$ sort input |groupby -w1
1,2013,1,3000	10,2013,10,8300	11,2013,11,7700	12,2013,12,1200	13,2014,1,8300	14,2014,2,3200	15,2014,3,7100	16,2014,4,3900	17,2014,5,4300	18,2014,6,7700	19,2014,7,9000
2,2013,2,8800	20,2014,8,2900	21,2014,9,8300	22,2014,10,6500	23,2014,11,1700	24,2014,12,3700
3,2013,3,9500
4,2013,4,5700
5,2013,5,2600
6,2013,6,4100
7,2013,7,9900
8,2013,8,900
9,2013,9,7100

$ sort -t, -k2 input | groupby -d, -s1 -f1
1,2013,1,3000	10,2013,10,8300	11,2013,11,7700	12,2013,12,1200	2,2013,2,8800	3,2013,3,9500	4,2013,4,5700	5,2013,5,2600	6,2013,6,4100	7,2013,7,9900	8,2013,8,900	9,2013,9,7100
13,2014,1,8300	22,2014,10,6500	23,2014,11,1700	24,2014,12,3700	14,2014,2,3200	15,2014,3,7100	16,2014,4,3900	17,2014,5,4300	18,2014,6,7700	19,2014,7,9000	20,2014,8,2900	21,2014,9,8300

$ sort -t, -k3 input | groupby -d, -s2 -f1
1,2013,1,3000	13,2014,1,8300
22,2014,10,6500	10,2013,10,8300
23,2014,11,1700	11,2013,11,7700
12,2013,12,1200	24,2014,12,3700
14,2014,2,3200	2,2013,2,8800
15,2014,3,7100	3,2013,3,9500
16,2014,4,3900	4,2013,4,5700
5,2013,5,2600	17,2014,5,4300
6,2013,6,4100	18,2014,6,7700
19,2014,7,9000	7,2013,7,9900
20,2014,8,2900	8,2013,8,900
9,2013,9,7100	21,2014,9,8300

$ sort -t, -k4 -n input | groupby -d, -s3 -f1
8,2013,8,900
12,2013,12,1200
23,2014,11,1700
5,2013,5,2600
20,2014,8,2900
1,2013,1,3000
14,2014,2,3200
24,2014,12,3700
16,2014,4,3900
6,2013,6,4100
17,2014,5,4300
4,2013,4,5700
22,2014,10,6500
15,2014,3,7100	9,2013,9,7100
11,2013,11,7700	18,2014,6,7700
10,2013,10,8300	13,2014,1,8300	21,2014,9,8300
2,2013,2,8800
19,2014,7,9000
3,2013,3,9500
7,2013,7,9900