Add dashboard performance trends

api/QuestionsApi/Services/RatingStore.cs

@@ -35,6 +35,12 @@ public class RatingStore
         };
     }
 
+    public async Task<UserRating> GetOrInitializeHistoryAsync(string userId, DateTime nowUtc)
+    {
+        var rating = await GetOrCreateUserRatingAsync(userId);
+        return await EnsureHistoryBaselineAsync(rating, nowUtc);
+    }
+
     public async Task<Dictionary<string, int>> GetQuestionElosAsync(IEnumerable<string> externalIds)
     {
         var ids = externalIds.Where(id => !string.IsNullOrWhiteSpace(id)).Distinct().ToList();
@@ -71,6 +77,7 @@ public class RatingStore
             };
         }
 
+        userRating = await EnsureHistoryBaselineAsync(userRating, DateTime.UtcNow);
         var actual = isCorrect ? 1.0 : 0.0;
         var userK = userRating.RatedQuestionCount < 20 ? 40 : 24;
         var questionK = questionRating.AttemptCount < 20 ? 32 : 24;
@@ -164,6 +171,26 @@ public class RatingStore
         return rating;
     }
 
+    private async Task<UserRating> EnsureHistoryBaselineAsync(UserRating rating, DateTime nowUtc)
+    {
+        if (rating.HistoryBaselineElo.HasValue && rating.HistoryStartedUtc.HasValue)
+            return rating;
+
+        var result = await _users.UpdateOneAsync(
+            value => value.UserId == rating.UserId && value.HistoryStartedUtc == null,
+            Builders<UserRating>.Update
+                .Set(value => value.HistoryBaselineElo, rating.OverallElo)
+                .Set(value => value.HistoryStartedUtc, nowUtc));
+        if (result.ModifiedCount > 0)
+        {
+            rating.HistoryBaselineElo = rating.OverallElo;
+            rating.HistoryStartedUtc = nowUtc;
+            return rating;
+        }
+
+        return await _users.Find(value => value.UserId == rating.UserId).FirstAsync();
+    }
+
     private async Task<QuestionRating> GetOrCreateQuestionRatingAsync(string questionExternalId)
     {
         var create = Builders<QuestionRating>.Update