SparkChartTooltipCallback<String>  typedef 
 
        SparkChartTooltipCallback<String> =
     String Function(TooltipFormatterDetails details)
     
    
Signature for the callback that returns the string value to override the default text format of trackball tooltip label.
Implementation
typedef SparkChartTooltipCallback<String> =
    String Function(TooltipFormatterDetails details);