this is suspicious as the acf function in stats should be using plot.acf
according to RDocumentation there are many many functions called 'acf' from many packages. Probably you are loading one and it is not behaving as you expect.
You could try explicitly using stats::acf(ihk_tr1,lag.max=48,ci=0.95) to avoid confusion from other loaded packages, or if you actually want help with the acf from one of those other packages, please tell us which.
to identify what loaded libraries contain acf in your session try
getAnywhere("acf")