Success... sort of... I used the [glide ___ secs to __ __ ] block to vary the time instead of the x value. Now the distances the dots travel are the same. (I do notice imprecision now in the timing! The times are not exact!) I think there is a lot work to do to model true pendulum movement :-D (This model has no friction ;-)
See comments below McScience_Lego's original project to see the background about this bug...
