I'm trying to draw a tangent plane for a general function $f(p,q)$ in Mac grapher. This requires computing the partial derivative of $f$ with respect to $p$ evaluated at point $(x_0,y_0)$. I have it in terms of $p$ and $q$ because of running into the same problem as this question. I cannot at all figure out the right notation to evaluate the partial derivative at a point.
I tried $dfx(p,q)=\frac{\partial f(x,y)}{\partial x}$ and then tried evaluating $dfx(x_0,y_0)$ but it is not a constant. How can I accomplish this?