A vertical line on the timeline marking a date (a release, a freeze, today...).
Markers are given to GanttEx.Live.Gantt with the markers attribute;
today_marker adds one for the current date.
iex> {:ok, marker} = GanttEx.Marker.new(%{date: "2025-03-01", text: "Release", color: "#e11d48"})
iex> {marker.date, marker.text, marker.color}
{~N[2025-03-01 00:00:00], "Release", "#e11d48"}
iex> GanttEx.Marker.new(text: "No date")
{:error, {:invalid, :date}}
Summary
Types
@type t() :: %GanttEx.Marker{ class: String.t() | nil, color: String.t() | nil, date: NaiveDateTime.t(), id: term(), text: String.t() }