Yes, if you actually mark the table w chalk behind the ob, it is a foul. That is in most rules I've read.
[1.39 Marking the Table
It is a foul if you intentionally mark the table in any way to assist you in executing any shot or future shot. Marking includes the deliberate placement of chalk or any other object at a specific point on a rail or cushion to aid the alignment of a shot, or placing any mark on any part of the table. The foul occurs at the moment you mark the table, regardless of whether you remove the mark or whether a shot is taken.