scaling/offset was only a suggestion as I don't really know how you have your display set up to show these + and - coordinate values.
The display data itself is just an array of memory locations laid out in row order from the top to bottom of the screen. You need to come up with an algorithm to translate an (x,y) data coordinate to a memory location based on the used display area.
In regards to what is done for the scaling/offset...
Suppose you have a 256x256 display area and you use a simple x,y data that is 1:1 with the pixels then if your x,y locations are limited to 1/2 the width and height of the screen pixels then you can just add 128 to the x and y coordinates to center the 0,0 position in the center of your 256x256 display (I'm sort of assuming that the values are stored in a 2's complement fashion so the range is -128 to +127). If those same x,y coordinates are -256 to +255 then you'll have to resort to scaling the data first (as the x,y can index into a 512x512 area) by dividing by 2 first to scale the x,y coordinate first, then shift the center. If you only have one axis that has negative values then you don't have to shift those coordinates.
Unfortunately I'm probably not explaining this well for you.