It's a bit ambiguous what it means to erase an abstract entity like "data". Usually, I would accept it to mean that there are no physical copies of the plain text remaining -- e.g. no copies in terms of bits on a hard disk or strokes of ink on paper. If you've overwritten the sole copy of the original plain text with the ciphertext on a hard disk, then I think it's fair to accept that as erasure of the original data.

Philosophically, of course, all strings of data that will ever be written exist in some abstract mathematical sense -- I can write a program that will eventually generate every single combination of bits that can be ever be stored on a given computer (should you care to wait long enough) including novels that don't exist yet (in the conventional sense that a human author hasn't put pen to paper), and the plaintext that was overwritten by the cryptography program.