It would be convenient if ImageToGraph accepted the mouse X, Y coordinates. Then there is no need to create an intermediate variable just to call ImageToGraph.
I added a small overloaded function to the source as a test, maybe it could be part of TAChart?
function ImageToGraph(const AX, AY: Integer): TDoublePoint; overload;
..
function TChart.ImageToGraph(const AX, AY: Integer): TDoublePoint;
begin
Result.X := XImageToGraph(AX);
Result.Y := YImageToGraph(AY);
end;