I may be mis-interpreting your query but surely your graph co-ordinates must comprise some mapping of y-coordinate to x-coordinate, probably simple x,y pairs, just look to see what the highest value of y is and then take the corresponding x value.
i've tried that, but the highest y-value is not neccessarily the highest point on the graph.
e.g the graph contains temperature readings which are taken by a machine e.g. every 0.5 degrees, the
graph that is printed out doesn't necessarily have coordinates for the peak on the graph.
;-)