Elliptic curve

How do I graph y^2 = x^3 +ax + b? I tried a parametric plot using x(t)=t^3+at+b, y(t)=t^2 but that doesn't look close. As a standard function y=sqrt(x^3 + ax + b) I only get half the plot. I can add the negative as a second plot, but is there a better way?