The POH graphs don't go above 7000ft and state "extrapolation of chart above 7000 ft is invalid". ...How can I make a flightplanning in this way?
There's your answer - don't plan on going to an airfield where the density altitude will exceed 7,000 ft. I've spent a lot of time flying in the Rocky Mountains and the PA28-140 (has it really got 160 hp?) would not be my choice. You'll find it frustrating at least, dangerous at worst at a high altitude take-off and mountains to avoid after take-off, or a go-round.