JavaScript 129 - 10 (30 character lines) = 99

r=Math.random;x=15+r()*2;z=(j=Array(29).join("_"))+"."+j;for(i=30;i--;)console.log(z.substr(x=(x+=r()<.5?-1:1)>29?29:x<0?0:x,30))

....5....0....5....0....5....0 <-- Ruler ___________.__________________ ____________._________________ _____________.________________ ______________._______________ _______________.______________ ________________._____________ _______________.______________ ________________._____________ _________________.____________ __________________.___________ ___________________.__________ __________________.___________ ___________________.__________ ____________________._________ ___________________.__________ __________________.___________ ___________________.__________ __________________.___________ ___________________.__________ __________________.___________ _________________.____________ ________________._____________ _______________.______________ ______________._______________ _______________.______________ ______________._______________ _______________.______________ ________________._____________ _______________.______________ ______________._______________