Using multiply y values was primarily motivated by stacked bar and area series. Application to line series came in later, but I see from your question that it does not seem to be complete...
Anyway, I'm going to show you three solutions how you could solve your problem. Suppose you have a csv file with one x column and 5 y columns (y1, ..., y5).
The simplest solution would be to add 5 series, each of them with YCount=1, and store (x,y1) in the first series, (x,y2) in the second, (x,y3) in the third etc. This, in fact, gives you the greatest flexibility, and you can even use a TChartListbox to display any combination of the series (--> project 1)
Of course, x is stored mulitple times in this solution. Although this multiple internal storage of a value very probably won't let you run out of memory, I agree it is not very elegant... So let's return to YCount=5; now each data point consists of a shared x and 5 y values here, and you need only a single series. In order to hav an individual color for each "sub-series" you must work with TChartStyles. Add a TChartStyles component to the chart and link it to the Styles property of the lineseries. Then add five Style elements (one for each series) and set the Pen.Color as you need for each sub-series. For all these lines which you do not want to see set the Pen.Style to psClear. The lines that you do want to see will have psSolid as Pen.Style (---> project2)
The disadvantage of this solution is that the hidden sub-series are still evaluated for calculating the chart extent. So, if one column of your csv file contains only very large values, and another column only very small values the sub-series with the small numbers will be almost flat.
This limitation is overcome by the third solution. There is no need to store the data of the csv file in a ChartlistSource at all, you can use an ordinary array of x-y1-y2-y3-y4-y5 records instead and use a TUserDefinedChartSource to assign the record element to the series. For this purpose you must write a handler for the OnGetChartDataItem event (---> project 3)