A data series is a set of related values that are plotted on a chart.
In Numbers, data series are represented differently in different kinds of charts:
Column charts and bar charts: A data series is represented by a series of bars in the same fill (color or texture).
Line charts (also called graphs): A data series is represented by a single line.
Area charts: A data series is represented by an area shape.
Pie charts: Only a single data set (the first data point in each series) is represented on the chart.
Scatter charts: Two columns of data are used to plot values for a single data series. Each pair of values determines the position of one data point.