There are several ways to get the x values at each intersection of the y-value. The simplest solution is to use the intersections() function from the file exchange (I show a similar demo in this answer, "method #2").
That function will return the (x,y) coordinates of the intersection points. Use the x values to plot vertical lines using xline().
% x is a vector of x-values at each intersection
arrayfun(@xline,x)
Or, if you're more comfortable with for-loops,
for i = 1:numel(x)
xline(x(i))
end
