As per https://ko.infragistics.com/community/forums/f/ultimate-ui-for-windows-forms/125359/stacked-area-series-on-timexaxis

Assume the following data source:

public AccountValueHistory()
{
this.Add(new AccountDateInfo { date = new DateTime(2023, 1, 1), Acct1 = 0, Acct2 = 30 });
this.Add(new AccountDateInfo { date = new DateTime(2023, 2, 16), Acct1 = -50, Acct2 = 30});
this.Add(new AccountDateInfo { date = new DateTime(2023, 2, 25), Acct1 = 50, Acct2 = 60 });
this.Add(new AccountDateInfo { date = new DateTime(2023, 2, 25), Acct1 = 70, Acct2 = 60 });
}

I'd have thought in almost all cases users would want to sum positive and negative values together.  At the very least I think this should be an option.  For example, if the graph shows the sum of two bank accounts, and one account is overdrawn and has -50 in it, and the other account has 30, your net worth at that time is -20.  It just doesn't make sense to display this as 30.