Pretty much every jigsaw on the market is incapable of sawing a straight line - but that isn't really what they are designed to do. They are actually designed to act like a cross between the old nest of saws, a turning (fine blade) bow saw and a coping saw - and none of thos can cut straight, either. If you want to cut straight, you need a circular saw. If you want a flat, straight cut you may also need a hand plane (e.g. a block plane) or a rail saw, But then your idea of straight isn't necessarilly what I'd find acceptable in a work situation!
The comments about blades made above are also very pertinent; you need to match the blade to the task carefully and there is quite a selection to choose from, but you also need to match the blade orbit (if orbital action is available) and speed to the task as well. One thing I've often seen is people trying to cut 38mm thick worktop from the top surface using a jigsaw with a downcut blade (T101BR), "because it will leave a smooth cut". That will also leave a bowed cut which is well off plumb - far better to use an up cut blade, but run the jigsaw on the underside[/] of the worktop, although granted that takes practice. Also not that once a blade has nbeen used to cut even medium tight curves it takes on a "set" to one side or the other, and will never sut straight again. So for (relatively) accurate cutting a fresh blade will be a must
Just to finish, I often scribe complex skirtings...
...with a jigsaw (the blade is a T244D)