I came across the sentence
混乱する気持ちもよーくわかる in my manga. I have translated it, but the use of the dash (which was vertical in the actual vertical text) stumped me for a bit (I thought it was よう at first, not よお). I was under the impression that a dash like that is only used in katakana, and in hirigana they use the character of the sound they want to extend. But that's not the case here.
So what does it mean when this happens? Is is a special case or exception, or is there some rule?
By the way, I ended up with
よーくわかる all together meaning "I know you..." (thanks, Google Translate, for being more useful than a dictionary for once), which seems to be right in this context (In this case, "I know you're feeling confused"). That's why I think it might be a special case. (Searching よお on it's own ended up with "trouble brought on by sins of forebears"...)