Assuming you have 4 cdf arrays, you can find the x-values as:
cdf1ind = find(cdf1>=.80,1,'first'); x1 = x(cdf1ind);
cdf2ind = find(cdf2>=.80,1,'first'); x2 = x(cdf2ind);
cdf3ind = find(cdf3>=.80,1,'first'); x3 = x(cdf3ind);
cdf4ind = find(cdf4>=.80,1,'first'); x4 = x(cdf4ind);
I would just plot the points instead of the line. For your existing figure:
hold on; plot(x1,cdf1(cdf1ind),'o');
To plot a dotted line on those x values:
hold on; line([x1,x1],[0,cdf1(cdf1ind)],'LineStyle','--');
To make the cdf curve smoother, try interp1() or something similar.
newcdf1 = interp1(x,cdf1,linspace(min(x),max(x),10*numel(x)));
The last question: that's totally upto you, what aspect of the idea you want to highlight.