I am going to Service Director > Service Level Manager > Plesk Domains > DOMAINNAME > Traffic statistics and switch view mode to "Hourly", but it shows only value for one hour a day.
By default Plesk collects statistics about traffic and disk usage once a day.
This behavior is by design and is caused by the fact that Plesk collects statistics about traffic and disk usage once a day by default.
OBAS stores statistics in its database based on a difference between values reported by Plesk and this difference is equal to zero most of the time (because values in Plesk database are not being changed) and real difference appear only when Plesk runs statistics accounting utility on a daily basis.
Therefore, corresponding statistics will be reflected in PBAS on the next hour after statistics was accounted by Plesk, all other periods of time will be empty because Plesk will report the same values of traffic and disk usage during all the period from the last statistics accounting till the next statistics accounting.