What are you plotting? Is it an xy scatter plot to which you are fitting a straight line?
I have produced the graph using the xy scatter yes, however I have also manually drawn the graph and the computer has drawn the straight line through the origin at '0', which is wrong, the graph should be at x = (-4, -2) (2, 7), I have used the equation to calculate the results also and know the manual drawn graph is correct.
So I am thinking now of forgetting the computer excell software programme, which I think is the wrong design for producing graphs with up to four quadrants.