If you were using nhc98, then you could use the following:

import NonStdProfile

f ... = do ... profile "point A" actionA profile "point B" actionB

The non-standard operation profile :: String -> a -> a places a labelled marker line on the profile graph at the time instant when the value is demanded.

I don't know whether ghc has anything similar?

No, but that strikes me as rather a good idea. I'll have a go at implementing it. Cheers, Simon