Straight diagonal lines are difficult on longarm. You would need to use a ruler as you quilt to get the lines straight. I do not know of any longarm that would have channel locks on the diagonal and I am not sure how you would put the quilt on the frame on the diagonal. Longarm do have stitch regulators. However, longarms are great for quilting. I don't know if you can get a computer program to do diagonal lines. If you think you want a longarm, I would see if I could find a shop that offers classes and also rents out time on one of those machines before deciding to buy.