Negative value would indicate "paying you to take them", which doesn't make any sense, unless there's a forfeit associated with having any.
Really, share prices should be on a logarithmic graph. You care whether your shares are now worth eg. twice as much or half as much as you paid for them originally. The actual number of shares that you could buy with a given amount of money isn't as interesting.
... which is ironic, since SystemD is what happens when someone comes up with a problem to fix your solution.