Straight line in putting
Now, I am going to say again that all swings in golf are an arc, and this includes the putter. Take a putter and hold it between your thumb and pointer finger and let it swing back and forth. Straight line, right? The problem is when you hold it, the shaft of the putter is vertical. It is not that way at address.
The putter shaft is on an angle. That means the putter has to swing slightly to the inside in the back swing and slightly inside on the through swing. The putter face would also slightly open in the back swing and slightly close in the through swing. Use an alignment stick as in the photo. Make strokes where the putter moves on an arc. It would not move straight back and through and the face would not stay square to the alignment stick.