Right, to decide if a point is above or below a line, find the lines equation in the form Y - MX - C = 0. M and C are constants to be found. It is not necessary that they be integers. (Ay, I've done too many maths papers. That is almost exactly how they would have a question phrased.)
This is fairly long winded, so bear with me. Assume the line is between points P and Q. Also assume the coordinate system is such that moving UP the plane increases Y and moving RIGHT increases X. I can't remember what Eufloria uses, but it shouldn't be much of a problem to tweak it to work.
To find the gradient of the line, take (Py-Qy) and divide by (Px-Qx). Call this result M.
Then, take (Py-MPx). Call this C.
You now have the information for the equation in the form Y = MX + C, where X and Y are coordinates of any point on the line.
Now this, when rearranged is equal to Y - MX - C = 0 . To check whether a point is above or below a line, insert the coordinates of the point into the above equation.
If it is satisfied, the point is ON the line.
If it is FALSE and Y - MX - C is a value LESS THAN 0, the point is below the line.
If it is FALSE and Y - MX - C is a value GREATER THAN 0, the point is above the line.
SPECIAL CASE: If (Px - Qx) is equal to zero, then the line is vertical, and calculating the gradient will result in (Py - Qy)/0. Not good. In this case, simply check what the X value of the point is in relation to the line and that will tell you which side of the line it is on. For horizontal lines (Py - Qy) = 0, there is not such error, and the long method can be used.