TimeFormatterSpec constructor
Creates a formatter for a particular granularity of data.
format DateFormat format string used to format non-transition ticks.
    The string is given to the dateTimeFactory to support i18n formatting.
transitionFormat DateFormat format string used to format transition
    ticks. Examples of transition ticks:
      Day ticks would have a transition tick at month boundaries.
      Hour ticks would have a transition tick at day boundaries.
      The first tick is typically a transition tick.
noonFormat DateFormat format string used only for formatting hours
    in the event that you want to format noon differently than other
    hours (ie: 10, 11, 12p, 1, 2, 3).
Implementation
const TimeFormatterSpec(
    {this.format, this.transitionFormat, this.noonFormat});