Thank you, sean_m and Paralus! I was in urgent need of your help! When debating on history it's necessary not only to use the correct chronology, but I think we'd distinguish accurately astronomy from astrology. When I say that Plutarch's date for Alexander's birth means the king was born in the constellation of Leo, I mean only that in 15 (or 20) July 356 B.C. the Sun occupied that position, according to astronomical calculation. Obviously this fact doesn't signify automatically that seers, priests, or individuals who were in Pella that day made Alexander's horoscope calculating ascendant, medium coeli, twelve houses, and so on. We must remember all the ancient evidences quoted by Taphoi are either generic, not having a precise astrological connotation (such as the lion mane hair of some portraits of Alexander, or the lion-scalp helmet Alexander wore in the Istambul sarcophagus - we must know that the "leontè", or leonine headgear was distinctive of Herakles even in pre-Alexandrian Macedonian coins of Philip II - ), either belong to later centuries (such as coins in Roman Alexandria, or the story of Olympias's lion sealed womb, from Plutarch, Life of Alexander (2, 4), or the passages of Pseudo-Callisthenes (1, 13, 3) and Julius Valerius (1, 7).
I do believe the Greeks had immense expertise of their own in the field of astronomy, but I don't know any Greek horoscope before the Hellenistic age. If someone has such evidence, I'll be glad to know. Till now arguably the first Greek horoscope we know is the one quoted by the stoic philosopher Chrysippus (280- 205 B.C.) according to Cicero, De fato 6, 12; 8, 16.