Think you have found a bug in Prime which you should report to PTC support.
The prime operator works for numerical evaluations but not for symbolical ones. The way the prime operator is documented it should work the same as the ordinary derivative operator, so its a bug if it doesn't.
Concerning the equal scale plot the answer is no. Generally plotting facilities in Prime are, spoken polite, rather mediocre. BTW you will see an ellipse anyway if you plot the functions you mentioned, but it may not have axis ration 2:1 unless you resize thge plot manually by visual judgement. A better example would be cos(t) over sin(t) which should be a perfect circle.