Código:
procedure TForm1.Chart1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
var xx, yy1, yy2: Double;
idx: TValuePos;
begin
{Screen coord -> Plot coord}
xx := Chart1.BottomAxis.CalcPosPoint(X);
{Binary search and interpolation (units in attachment) }
yy1 := ValueAt(ArrayAdapter(Series1, 'x'), ArrayAdapter(Series1, 'y'), xx);
{or}
if BinSearch(ArrayAdapter(Series2, 'x'), xx, idx)
then yy2 := ValueAt(ArrayAdapter(Series2, 'y'), idx)
else yy2 := NaN; // out of range
Caption := Format('%.3f %.3f %.3f', [xx, yy1, yy2]);
end;