This example visualizes circular trajectories for the dynamical system \(\dot{x}=xy,\dot{y}=(1-x^2-y^2)/2\). A click on the phase plane draws the unique circular path determined by the chosen initial condition, with a red center marker and a blue dot tracking the motion. The D3.js code renders an SVG canvas with axes, tick labels, and a set of precomputed trajectory circles in light gray, while user interaction dynamically adds a highlighted trajectory. The implementation uses `d3.select`, `d3.scaleLinear`, and SVG element `append` calls to build the plot, updating the circle’s position and radius via `attr` on mouse events.