In ChartDirector terminology, a data set is a series of data points.
In your chart, it looks like there is only one data set. The data set contains 7 data points.
For an area chart, if there area more than one data sets in a layer, you would need to specify how the data sets are combined in the layer. The following is an example of an area chart with 3 data sets, which are combined by stacking up.
Are you referring to the number of data points in a series? The number of data points are just the size of your data array. You can iterate the x series by using Layer.getXPosition. If you iterate beyond the end of the x series, it will return NoValue. You can also use Layer.getNearestXValue to find the x value nearest to the left and right edges of your chart, which is typically the first and last x data value.
Re: ChartDirector AreaChart not producing a Layer.GetDataSetCount