Nice! In case anyone is wondering what the rotation is based on, why you need four signals into the sin-cos potentiometer, and why you need to do that addition (with resistors) and difference (with opamp), is that
$$\begin{aligned}
x^\prime &= x \cos \theta - y \sin \theta \\
y^\prime &= y \cos \theta + x \sin \theta \\
\end{aligned}$$
is how you rotate 2D coordinates around origin (0,0), with \$(x, y)\$ being the coordinate signals from the Arduino+DAC board, and \$(x^\prime, y^\prime)\$ the coordinate signals provided to the XY scope.
(Sorry for pointing out the obvious! I just thought it might help clarify how that works, for those not familiar with rotations and 2D math. I like 2D and 3D math myself, from linear algebra to projections to raycasting and raytracing.)