For simple closed curve(circle). Yes. please use polygon element.
For ths above case (draw area between two vertical lines ,for projectile motion etc...) :
Draw those curves with Analytic Curve, and fill the region you want with polygon.
You can draw several curves with one analyticCurve element.
Define an array as values for parameter, and define the name used in your equations.
Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=427.0]Blackbody radiation curves for different temperatures[/url]. It was plotted with one AnalyticCurve element.