Hmm, if that doesn't work, try using some physics equations! To get you started, observe that during your trajectory, your object's coordinates will change as follows (from physics equation for an object under constant acceleration i.e gravity):
y(time) = initialYSpeed * time + ( acceleration * time^2 ) / 2
x(time) = initialXSpeed * time
To show your trajectory, just run a loop for (time = 0) to (time = 10) or something similar, and create a circle at your calculated y and x, using loopindex as 'time' in those equations. Experiment until you get the result you are looking for. You'd be best off using small 'circles' that make up the path instead of wanting a continuous line. You can either destroy all the circles every tick, or use a set amount of circles and position them each time. I can try doing an implementation if that will help. But I'll only be able to get to it in a couple days.