Old post, but...In Maple 17, you can define the function the way you did (i.e., without units), and then include the appropriate units in just the function call that is inside the plot command. Example:

(Note: The following is in 2-d math format.)

y₀ := 1.5[m]; v_0y := 5[m]/[s]; g := 9.807[m]/[s]²
y := y0 + v_0yt - (1/2)gt²


Then to graph this function, put the units [s] inside the function y(t); that is, enter y(t[s]):

plot(y(t[s]), t = 0..2)

That'll do it.

Cheers,

K.