Witchmail can trigger on Graves’ Jar of Dead while it is on charge cooldown, but it does nothing. Jar count and charge cooldown are unaffected. It just prevents another ability from having its CD reduced.
Witchmail will not trigger on Jar of Dead when it off cooldown, regardless of charge count.
Video: 4/5 Witchmail procs do nothing, because they proc on Jar of Dead.