graph sum question..

https://i34.tinypic.com/bi10z9.gif
i cant understand how they sum those three graphs.
when we sum graph 2 and 3 we get graph 3 lifted by a certain number.
but when we add graph 1 to the result
on point where the current is 0 the voltage is from 0 to -infinity.
before that current 0 that is no points,so on that section it cannot affect the
result graph
but it does .
why?

in the first graph i < 0, v = undefined

undefined + 10 + 10 i = undefined

thanks :")