using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var entries = new EventRecord[]{

            new EventRecord{DateAdded = DateTime.Parse("2006-12-30 00:38:54"), Event="click", Category = "externalWebsite"},

            new EventRecord{DateAdded = DateTime.Parse("2006-07-20 00:36:44"), Event="click", Category = "social"},

            new EventRecord{DateAdded = DateTime.Parse("2006-09-20 00:36:44"), Event="click", Category = "social"},

            new EventRecord{DateAdded = DateTime.Parse("2006-09-22 00:12:34"), Event="load", Category = "profile"}, 

        };

        var dateRangeStart = DateTime.Parse("2006-01-01");

        var dateRangeEnd = DateTime.Parse("2007-01-01");

        var monthRange = MonthsBetween(dateRangeStart,dateRangeEnd);

        var results = entries.Where(e => e.DateAdded>=dateRangeStart && e.DateAdded<dateRangeEnd)

            .GroupBy(x => new {

                x.Event,

                x.Category

            }).Select(g => new {

                g.Key.Event,

                g.Key.Category,

                Series = g.GroupBy(x => new {x.DateAdded.Month, x.DateAdded.Year})

                    .Select(i => new{

                        i.Key.Month,

                        i.Key.Year,

                        Count = i.Count()

                    })                  

                    .ToArray()

            });

​

        foreach(var item in results)

        {

            Console.WriteLine($"Event:{item.Event} Category:{item.Category}");

            var joinedSeries = from month in monthRange

                                join serie in item.Series

                                on new{month.Year, month.Month} equals new {serie.Year, serie.Month} into joined

                                from data in joined.DefaultIfEmpty()

                                select new {

                                    Month = data == null ? month.Month : data.Month,

                                    Year = data == null ? month.Year : data.Year,

                                    Count = data == null ? 0 : data.Count

                                };

            foreach(var serie in joinedSeries)

                Console.WriteLine($"\t{CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(serie.Month)}{serie.Year} Count={serie.Count}");

        }

    }

    private static IEnumerable<(int Month, int Year)> MonthsBetween(

            DateTime startDate,

            DateTime endDate)

    {

        DateTime iterator;

        DateTime limit;

​

        if (endDate > startDate)

        {

            iterator = new DateTime(startDate.Year, startDate.Month, 1);

            limit = endDate;

        }

        else

        {

            iterator = new DateTime(endDate.Year, endDate.Month, 1);

            limit = startDate;

        }

​

        var dateTimeFormat = CultureInfo.CurrentCulture.DateTimeFormat;

        while (iterator < limit)

        {

            yield return (iterator.Month,iterator.Year);                

            iterator = iterator.AddMonths(1);

        }

    }

}

​

public class EventRecord {

   public DateTime DateAdded {get;set;}

   public string Event {get;set;}

   public string Category {get;set;}

}