ST The Force Awakens Planets: A Guide

We don't really know what the yellow line is. I'm just speculating. The line could indicate known systems that have Jedi Temples and could mark the journey of the scouting parties searching for him.

Or since the map was held by R2, it could have been the route Skywalker took.

It's possible that in searching for the first Jedi Temple, Luke was mapping out possible systems where it could be (the yellow line), and eventually he found the first Temple, removed it and the surrounding sector from R2s map, so that nobody could find him. Kind of like Dooku deleting Kamino from the Jedi archive.

R2 then went into low power mode. BB-8 shows up with his fragment that Luke removed, but the Resistance does not have a map to cross reference with. And only when R2 awakes can they find Luke.

Meanwhile, Kylo Ren mentions having a map of the Galaxy retrieved from the Imperial archives. So they didn't need R2, just BB-8.

I believe Abrams even mentioned that the idea they were going with was that the map R2 has is the same one the First Order had, and that he retrieved it from the Empire years earlier aboard the Death Star.

Admittedly, there is still a suspension of disbelief required to accept that R2 had the only map of the Galaxy that the Resistance could use. And why couldn't literally any other astromech help them? Since astronavigation is what they do.

I do believe this is just a case of a poorly planned plot point used to drive the story forward.

Unless we are meant to believe that the Empire had better maps of the Galaxy than the successor governments.

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you said -Meanwhile, Kylo Ren mentions having a map of the Galaxy retrieved from the Imperial archives. So they didn't need R2, just BB-8..............and by that Ren just reading Rey's mind they should have been able to pinpoint the temples location and a sarce Jedi place like that would make rey's "force-sent" tingle like crazy (because the jedi found illum by a pull in the force) so it should be easy for Ren to find this planet if not Snoke should have and if not just go to the Chiss(they known every thing in the UR) and hey Even Snoke may be Chiss (not all chiss have blue skin, some have silver/gray skin)

 

Luke route map i made