[R] Variogram with time series

[consentino@infinito.it]

Hello,
   
I'm using a dataset with unequally spaced time series
and I'd want to know if there is in R some functions in
order to calculate the autocorrelation function, because
acf() in stats package cannot calculate it, because I
have many missing data.
   
And if so, is it necessary to organize dataset in ts
structure or is it possible to consider it like a simple
vector?
   
Thanks in advance

Fabrizio Consentino