Hour ( YourTimeStampField )
will return the Hour. (on 24 hour clock)
You can use this expression in a calculation field and sort by this field to group your records by Hour.
You can use a field of type summary to count your records. Specify the "count of" option and select a field to "count" that is never empty--such as your timestamp field.
You can sort your records by the Hour calculation field and use the Summary field as your Y value. Be sure to use the "groups of records when sorted" option.
Another calcuation field can use GetAsDate instead of Hour to extract the date so that you can chart your records by Day. You can use the same summary field for both charts, but the second chart would be sorted by this date calculation instead of the Hour calculation to group records by date instead of by Hour.