Fernando,
I don't exactly have additional information, but I think that your timeline should include more information about dBASE II, III, III PLUS, IV, and even V. Also, while I never used it, it seems to me that Clipper had an important impact in the xBASE world. Major versions of Clipper should be included.
Hilmar.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)