This sample illustrates Facebook users in a chart for different countries over several years. In a line-based series, data points can be annotated using symbols.
Each data point in a series can be represented by a symbol as a marker. You can customize the shape, size, and color of the symbol with the ChartMarker class.
Tooltip is enabled in this example. To see the tooltip in action, hover over a point or tap on a point in touch-enabled devices.
More information about the marker can be found in this documentation section.