The best free alternative to GnuPlot is GNU Octave, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to GnuPlot and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to GnuPlot are GeoGebra (Free, Open Source), Matplotlib (Free, Open Source), Desmos (Free) and Microsoft Mathematics (Free).
Graph toy is an interactive graph visualizer using mathematical functions. Based on the original Graphtoy by Inigo Quilez, it provides some helpful additions including an easy way to share graph links.
Functy is a 3D graph drawing package. It allows Cartesian, spherical and parametric curve functions to be plotted and altered quickly and easily. This immediacy and the vivid results promote fun exploration of 3D function.