I believe the panel mounts don't - or they show it as a very short line only.
Obviously this is really useful because in just about all phases of flight one is holding a heading, and one adjusts that periodically to line up with the GPS track - the magenta line.
When I see pictures like this I wonder if these do it...
Yes, they do. The Garmin 695 shows a barber line with small orthogonal lines indicating where you will be in 1, 2, 3, 4 and 5 minutes. Very useful. The TB10 I currently fly only has a working HDG mode for the AP and I keep adjusting the heading bug so that the track line on my 695 is aligned with my flight planned route. That is very easy.
You can see on the picture that I'm 4 degrees off to the left, in addition to the track line I look at BRG and TRK on the 695 and make sure they're the same.
