The figure is likely produced using fplot. The appropriate object handle is of type "ImplicitFunctionLine".
f=openfig('imp.fig');
f.Children.Children
h=findobj(f,'type','ImplicitFunctionLine')
% Retrieve the data
x=get(h,'XData');
y=get(h,'YData');
% Retrieve the function (you can obtain the data by evaluating the
% function)
fcn = h.Function
xr = f.Children.Children.XRange
yr = f.Children.Children.YRange
figure
plot(x, y)
