Sometimes (not always) its a good idea to use Mathcads Quickplot feature and let it chose the step width automatically:
tt is not defined anywhere.
Is there a reason for the strange stepwith 1/w you defined and why you don't start the parameter at zero?
Your sine vk has a period of somewhat lower than 10, so when you run t from 0 to L1 (approx. 200) then you are looping through your "8"-curve 20 times.
Your range t2 starts from approx 150 so the curve still is looped through 5 times. This and the fact that you had chosen a large stepwidth and because Mathcad connects the points calculated by straight lines created the effect you saw.
