I have the following two graphs:
$$f(x)=(43700 - x)/2$$ $$g(y)=-16150 + 100 y - 2 (-19000 + 100 y)$$
The functions $f$ and $g$ show the liquidation price of a margin trading position for a commodity with price $x$ (with the single underlying collateral in the commodity with price $x$ or with price $y$). So that when the price of the commodity goes down, the liquidation price of the position goes up.
I would like to build a 3D graph of the liquidation price $z(x,y)$, when a single commodity $x$ has underlying collateral in both the commodity with price $x$ and the commodity with price $y$. This happens when a margin trading position is open for commodity $X$, but has underlying commodities $X$ and $Y$ as the collateral. Thus the liquidation price depends on both $x$ and $y$, and not just on either one of them.
Of course, simply combining $f$ and $g$ in the fashion $z=f+g$ won't work, since the liquidation price depends on the price of two collateral commodities, $x$ and $y$, but the trading position is denominated in just the commodity $x$.
Long story short, how does one build the graph of the function which is similar to $f(x)$, but also depends on $y$ as in $g(y)$? The idea is to combine the variables $x$ and $y$ in the two functions to make a single 3D graph dependent on both variables.