Maybe it won't hurt if I mention, that this solution seems quite confusing (maybe not intuitive or nonstandard) to me.
Series1.index:= 0 seems to me like „delete series 0 and put series.index in its place", but in fact it does „increment the number of all series and put series.index in position 0“.
I suppose this is the reason why @wp did not expect that the index was writeable.
I if the index property of list boxes is writeable, just to find out that they do not have such a property.
As far as I remember, Lazarus's functionality about arrays is quite limited, if you think (maybe inserting series is really a rare case) it might be useful, I can try to spend some time to check how situation is.