if given a f(x) in y=log(f(x)), use where it equals 0 to draw verticle asymptotes, and where f(x)<0 y=log(f(x)) does not exist so you dont draw anything for those x values, also when f(x)=1 log(f(x))=0
keep in mind that if there is a minimum turning point on the x axis (such as f(x)=x^2 at 0) there a vertical asymptote with values aproaching from y=-infinty from both sides