hispls 5,000+ posts
CarAudio.com Veteran
It has been a long time but the Madisound type stuff that people over at DIYMA typically use usually gets enough notice by reviewers with the measuring rooms and equipment to post those kinds of graphs.That really interests me. Do you have anything that you've seen that stands out? As far as how they engineer that in? It seems to me it would largely be around cone angular design, for "dispersion", if you will.
A bit of looking on Google turned up this as an example:
Scan-Speak D2904/7100 Tweeter
www.speakerdesign.net
You can see that 30-45 degrees off axis seems to be the sweet spot for the flattest response.
As to how one goes about cone geometry or other design factors that effect this it's above my pay grade, but I strongly suspect the popular premium car audio brands take this into consideration since they know that the vast majority of people buying them are just going to drop them into factory locations and listen off-axis.