How can I interpolate a point in a x,y graph given only one of these values and being the point on the line?

I have a sets of graphs where I want to show 2 points on each plot. One on .5 (y value) and the other on .9 (also y value)

I have tried

plot(x,y)

hold on

plot(x(),y(.5),'r*')

But did not work out.

Any ideas?