The proportion $x_i$ of $\sum_i x_i$ can be easily determined and interpreted using the formula $\frac{x_i}{\sum_i x_i}$ if all $x_i$ are positive.
But now I have positive as well as negative $x_i$. How to proceed then? How should the result be interpreted? For signed $x_i$ it can also happen that the denominator becomes zero. What would be the right approach?