Here is a theory (there would be several other ways I can thing of too):
Calculate the hour separately and the date separately.
Create a summary field called min time which gives you min of your timestamp
Sort by date and hour and timestamp.
Create a calculation getsummary(min time, hour)
Create another calculation which says if (Timestamp = min time; Timestamp;'') (you might do it on a Time field and replace the value so it doesn't recalculate)
Now you have records marked for the first entry for each hour. You should be able to find those records and chart them.
That's a time-interval sort of approach... so you are not actually charting the records between. You could use a relationship back to the same date and hour then chart the related measures as averages.