addplot [thick,black]
table[x=freq,y=mod2,col sep=space] {file.csv};
coordinate (A) at (axis cs:0,-74.927351816709300);
coordinate (O1) at (rel axis cs:0,0);
coordinate (O2) at (rel axis cs:1,0);
draw [red,sharp plot,dashed] (A -| O1) -- (A -| O2);