I think what you are missing is a practical way to use the equation of the line (y=C+mx) and for the distance of two points on the line (square root of the square on the hypoteneuse) to generate the value of x2 that you require to plug into your graphics package to generate a value of y2 that is 100 units from y1 along the line. 1contract's algebra is correct. So if you put it into a spreadsheet, you can solve iteratively for a close approximation to the desired dx (of course, dx = dy/m, but in practice if your dy is itself determined by plugging in known values of x, you don't know dy independently of dx, so this neat fact does not help). Attached is a spreadsheet which enables you to plug values for x and y into the linear equation which allow you to solve for the distance along the line being as close to 100 as you like.