TooltipTextFormatterCallback typedef
Signature for formatting or changing the whole tooltip label text.
Implementation
typedef TooltipTextFormatterCallback = String Function(
/// actualValue will be either [DateTime] or [double]
/// based on given [values].
dynamic actualValue,
/// If the actual value is [double], it is formatted by [numberFormat] and
/// if the actual value is [DateTime], it is formatted by [dateFormat].
String formattedText);