I asked the same exact question in Stack Overflow but I'm afraid it could be OT therefore I'm asking it also here.
I was wondering on how I could plot this curve $\left(x_3(t)-\frac{1}{\omega^2b}\right)^2+\left(\frac{x_4(t)+ \gamma\left(x_3(t)- \frac{1}{\omega^2b}\right)}{\delta}\right)^2-\frac{1}{\omega^4b^2}\left(1+\frac{\gamma^2}{\delta^2}\right)e^{-2 \gamma t}=0$ in MatLab. Both $x_3(t)$ and $x_4(t)$ are well defined and are of the form $Ae^{-t}cos(t)$.
I tried both 'fplot' and 'plot' but I kept getting this : 'Input must be a function handle or symbolic function. '. If I use 'fimplicit3' I get an empty graph.