plotting issue with timestamps

I have a dataset with transactions and a timestamp at which they occoured 
during a day.  The time stamp is in the format YYYY/MM/DD hh:mm:ss.  I would 
like to plot a timeseries of the transactions to see if there is a 
particular time in the day when there is a spike in transactions.  Ofcourse 
the YYYY/MM/DD can be dropped since I am monitoring activity for the day and 
the actual date is unimportant.

Can anyone give me some direction on this.  I could possibly build a 
frequency table but not sure how to plot against the timestamp in the format 
above.  Any help would be appreciated.

-Dhiren