Adding new lines to multi-line chart

[sts8500man]

I have a line chart graphing 4 data series – dollar amount – Y axis and date – X axis. I can add a data series to this chart as long as the beginning date of the data series to be added starts on the same date as the lines already graphed. Unfortunately, when I try to put a data series on the same chart that does not begin with the same date as those previously graphed, the line for it does not appear in the chart. Can someone tell me why this is the case and how I can remedy that situation?