x,y,t = var('x,y,t')
f = lambda x,y: x*abs(y)/sqrt(x^2+y^2) if (x,y)<>(0,0) else 0
a,b = 1,6
e = 0.1
parametric_plot3d( [b*t, -a*t, f(b*t,-a*t)], [t,-e,e], color="yellow", size=15, plot_points=200 )