How to add a reference horizontal line to an existing plot

I want to add a reference horizontal line at about -86 db in my plot.How can I do it

Answer 1

[TRISHITA BANERJEE]

        hline = refline([0 -80]);  % highlight reference thersold at -80 DB
        hline.Color = 'r';

Answer 2

[KSSV]

Check the below demo code:

x = linspace(0,2*pi) ;
y = sin(x) ;
plot(x,y)

% reference line at x = 0.6
xi = linspace(min(x),max(x)) ;
yi = 0.6*ones(size(xi)) ;

hold on
plot(xi,yi) ;