I am looking to graph autocorrelations using graph combine via the following code:

use "`root_folder'/Data/S&P Pre 2011 Sample.dta"
tsset date
ac vwretd, name(no1, replace)
ac ewretd, name(no2, replace)
graph combine (no1 no2), name(combine1, replace), cols(2), xsize(6.5) ysize (10) title("Autocorrelation Function: Pre 2011")

use "`root_folder'/Data/S&P Post 2011 Sample.dta"
tsset date
ac vwretd, name(no3, replace)
ac ewretd, name(no4, replace)
graph combine (no3 no4), name(combine2, replace), cols(2), xsize(6.5) ysize (10) title("Autocorrelation Function: Post 2011")

graph combine (combine1 combine2), name(combine3, replace), rows(2) cols(1) title("Autocorrelation Functions")

The code delivers the graphs side by side. I want them to presented one below the other. I am wondering why this code doesn't work?