Blazorise Gantt: Timeline

@code {
    private bool autoExpandView = true;

    private DateOnly selectedDate = DateOnly.FromDateTime( DateTime.Today.AddMonths( -3 ) );

    private GanttView selectedView = GanttView.Week;

    private List<TaskItem> tasks = CreateTasks();

    private static List<TaskItem> CreateTasks()
    {
        DateTime rangeStart = new DateTime( DateTime.Today.Year, DateTime.Today.Month, 1 ).AddDays( -4 );

        List<TaskItem> result = new()
        {
            new() { Id = "1", Title = "Auto-expand release", Start = rangeStart, End = rangeStart.AddDays( 41 ) },
            new() { Id = "2", ParentId = "1", Title = "Design phase", Start = rangeStart, End = rangeStart.AddDays( 8 ) },
            new() { Id = "3", ParentId = "1", Title = "Implementation", Start = rangeStart.AddDays( 7 ), End = rangeStart.AddDays( 28 ) },
            new() { Id = "4", ParentId = "3", Title = "Backend services", Start = rangeStart.AddDays( 9 ), End = rangeStart.AddDays( 18 ) },
            new() { Id = "5", ParentId = "3", Title = "Frontend polish", Start = rangeStart.AddDays( 16 ), End = rangeStart.AddDays( 31 ) },
            new() { Id = "6", ParentId = "1", Title = "Validation and rollout", Start = rangeStart.AddDays( 30 ), End = rangeStart.AddDays( 41 ) },
        };

        foreach ( TaskItem item in result )
        {
            item.Duration = Math.Max( 1, (int)Math.Ceiling( ( item.End - item.Start ).TotalDays ) );
        }

        return result;
    }

    public class TaskItem
    {
        public string Id { get; set; }

        public string ParentId { get; set; }

        public string Title { get; set; }

        public DateTime Start { get; set; }

        public DateTime End { get; set; }

        public int Duration { get; set; }
    }
}

@code {
    private DateOnly selectedDate = DateOnly.FromDateTime( DateTime.Today );

    private List<TaskItem> tasks = CreateTasks();

    private List<GanttMilestone> milestones = CreateMilestones();

    private static List<TaskItem> CreateTasks()
    {
        DateTime baseDate = GetStartOfWeek( DateTime.Today, DayOfWeek.Monday );

        List<TaskItem> result = new()
        {
            new() { Id = "1", Title = "Milestone release", Start = baseDate, End = baseDate.AddDays( 14 ) },
            new() { Id = "2", ParentId = "1", Title = "Planning", Start = baseDate, End = baseDate.AddDays( 2 ) },
            new() { Id = "3", ParentId = "1", Title = "Design", Start = baseDate.AddDays( 2 ), End = baseDate.AddDays( 6 ) },
            new() { Id = "4", ParentId = "1", Title = "Development", Start = baseDate.AddDays( 5 ), End = baseDate.AddDays( 11 ) },
            new() { Id = "5", ParentId = "1", Title = "Validation", Start = baseDate.AddDays( 10 ), End = baseDate.AddDays( 13 ) },
            new() { Id = "6", ParentId = "1", Title = "Rollout", Start = baseDate.AddDays( 13 ), End = baseDate.AddDays( 14 ) },
        };

        foreach ( TaskItem item in result )
        {
            item.Duration = Math.Max( 1, (int)Math.Ceiling( ( item.End - item.Start ).TotalDays ) );
        }

        return result;
    }

    private static List<GanttMilestone> CreateMilestones()
    {
        DateTime baseDate = GetStartOfWeek( DateTime.Today, DayOfWeek.Monday );

        return new()
        {
            new() { Date = baseDate.AddDays( 2 ), Title = "Kickoff" },
            new() { Date = baseDate.AddDays( 6 ), Title = "Design approved" },
            new() { Date = baseDate.AddDays( 11 ), Title = "Beta" },
            new() { Date = baseDate.AddDays( 14 ), Title = "Launch", TextColor = TextColor.Danger, LineStyle = GanttMilestoneLineStyle.Solid },
        };
    }

    private static void ApplyMilestoneStyling( GanttMilestone milestone, GanttMilestoneStyling styling )
    {
        if ( milestone.Title == "Beta" )
        {
            styling.TextColor = TextColor.Warning;
            styling.LineStyle = GanttMilestoneLineStyle.Dotted;
        }
    }

    private static Color GetMilestoneBadgeColor( GanttMilestone milestone )
    {
        return milestone.Title switch
        {
            "Launch" => Color.Danger,
            "Beta" => Color.Warning,
            _ => Color.Primary,
        };
    }

    private static DateTime GetStartOfWeek( DateTime value, DayOfWeek firstDayOfWeek )
    {
        int diff = ( 7 + ( value.DayOfWeek - firstDayOfWeek ) ) % 7;

        return value.Date.AddDays( -diff );
    }

    public class TaskItem
    {
        public string Id { get; set; }

        public string ParentId { get; set; }

        public string Title { get; set; }

        public DateTime Start { get; set; }

        public DateTime End { get; set; }

        public int Duration { get; set; }
    }
}