The format set by the DataFormatString property from the TooltipsAppearance element is a common for the whole series and cannot be unique for series items within the same series. The exception is the Bubble Series which has a ToolTip property exposed by each series item, so that you can specify a tooltip programmatically for each series item.
What you can do, however, in order to specify different tooltips/labels for series items is to use the ClientTemplate functionality. For more information on how to use ClientTemplate for tooltips and labels please refer to this
online demo as well as to this
Note also the ClientTemplate functionality is available only for a databound chart. That means if your chart is created programmatically then you can recreate your current logic, so that instead of creating items programmatically create a datasource which can be bound to the chart.
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