Ageratum is one of my favorites but it's not the same thing as the NOID in my front garden that I am comparing to your plant. Mine had long-lasting spikes of purplish flowers, not exactly the same color as the Veronica 'Goodness Grows' which was growing just behind it. I remember that the flower spikes got top-heavy and fell over.
(Sorry that the color in my photos is sort of washed-out looking.)
Here is your plant:
Here are pics of mine; first from April 2013, the rest are from today:
Here is a shot of the fine hairs on the round stems; the hairs are also on the leaves.
We may have to wait until both plants start to bloom to get the ID.
Note: The perennial Ageratum that I introduced into my yard last year is NOT a favorite of mine - all Ageratums are not created equally - I prefer the mild-mannered annual type.
