Where did Plato get his data? ?

History was his guide.  He was Athenian and saw it happen first hand.  Athens slipped into tyranny multiple times:

Cylon, 632 BC (stoned)   

Pisistratus, 561 BC, 559-556 BC and 546-528 BC   

Hipparchus, 527-514 BC (murdered)   

Hippias, 527-510 BC   

Theramenes, Critias, and Charicles leading members of the Thirty Tyrants 404-403

Lachares, 300-294 BC   

Aristion, 88-86 BC (executed)

Logic, the source of all data. 

Knowledge, not data.

It’s obvious. The system of governance never remains the same forever 

Yes, they did. The Greeks invented nothing except man-boy love