MIL-HDBK-5J (p.5-66) gives a full range stress-strain curve for annealed 6Al-4V sheet at RT conditions. The difference in mechanical properties between annealed sheet and annealed 5/8" plate are probably less than 5% (since they are both wrought forms), so maybe that is adequate for what you need.
Thanks, Terry, but I already have the curve for sheet. Sheet is stronger than plate (see elsewhere in HDBK-5/MMPDS) so the sheet curve is higher than the plate one. I proposed shifting the top part downwards to match the plate Ftu, but our material man was not in favour...