As graph matrices?

I would like to please guide me on this question:
Let $S_+$ denote the set of semi positive definite matrices in $\mathbb{R}^{2\times 2}$ is known that $S_+\subseteq Sym \simeq\mathbb{R}^{3}$ ,wherein $Sym$ are the matrices symmetric. But Is it possible to give a geometric interpretation of $S_+$ in $\mathbb{R}^{3}$ ? Can be graphed?
Thank you very much for your attention, any suggestions are welcome.