Solution 1:

"Elapsed" is a participle a.k.a. verbal adjective.

As an attributive adjective, it can come either before or after the noun, as long as it is next to it.

And since you're only using it in a label (not in a sentence), both "elapsed time" and "time elapsed" are correct and possible.

Solution 2:

It may be just a matter of style, but I'd prefer "Elapsed Time". "Elapsed" is an adjective, which normally preceeds the noun. For a plot label, either is fine.