A Lissajous curve, that can be expressed in parametric form as:

${\displaystyle x=A\sin(at+\delta ),\quad y=B\sin(bt)}$

with ${\displaystyle a=1}$, ${\displaystyle b=2}$ and ${\displaystyle \delta =\pi /2}$. I created it with the following GNUplot code:

set samples 5000
set terminal svg
set output "Lissajous_curve_1by2.svg"
# plotting within a square
set size square

# remove most of information from the picture
set noxtic
set noytic
set key off
unset border

# plot
set parametric
a=1
b=2
delta=pi/2
plot [0:2*pi] sin(a*t + delta),sin(b*t) with lines linewidth 2