I have a very large file of 2.3 million hospital admissions, and am using stata v 13.1 on a MacBook Pro
I want to create three bar charts of the hospital cost of complications for hip replacement, using three different icd codes for the hip procedures. I want to then  combine the three in one graph. 
This seemed straightforward with the following loop:
gen i=1
foreach v of varlist HIP1824Px HIP1836Px HIP1819Px {
 graph bar total_episd_amt if LOS>0 & hospital_group=="" & separation_fy=="2010/11", over (COMPTOT, relabel(2 "Complication" 1 "NoComplication")) over(`v', relabel(2 "`v'"  1 "  Other admissions")) title ("Mean episode amount overnight stays, public hospitals 2010/11") subtitle ("`v' with/without complications") name(HIP`i')
 replace i=i+1
 }
graph combine HIP1 HIP2 HIP3,cols(1)
The error message is : HIP1 is not a memory graph
I have tried various combinations of the saving and name conventions, including double quotation marks around the `i' in the name, but none work.
Any  ideas, please?