Fix Lite perfmon chart bugs and Dashboard ScottPlot crash handling (#544, #545)

Lite perfmon fixes (#544):
- Aggregate chart query by collection_time (SUM across instance_name)
  so multi-instance counters show one clean line instead of jagged duplicates
- Add gap detection to DeltaCalculator — if >5min since last cached value,
  return 0 delta instead of inflated value spanning the entire gap
- Seed timestamps from DuckDB so gap detection works after app restarts
- Fix sample_interval_seconds from hardcoded 600 to 60 (actual interval)

Dashboard fix (#545):
- Wrap ServerTab construction in try-catch with user-friendly error message
  pointing to VC++ Redistributable download for SkiaSharp failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: erikdarlingdata

Dashboard/MainWindow.xaml.cs

@@ -502,7 +502,26 @@ private async Task OpenServerTabAsync(ServerConnection server)
             var utcOffset = connStatus.UtcOffsetMinutes ?? (int)TimeZoneInfo.Local.GetUtcOffset(DateTime.UtcNow).TotalMinutes;
             Helpers.ServerTimeHelper.UtcOffsetMinutes = utcOffset;
 
-            var serverTab = new ServerTab(server, utcOffset);
+            ServerTab serverTab;
+            try
+            {
+                serverTab = new ServerTab(server, utcOffset);
+            }
+            catch (Exception ex)
+            {
+                var inner = ex.InnerException?.Message ?? ex.Message;
+                System.Windows.MessageBox.Show(
+                    $"Failed to open server tab for '{server.DisplayName}'.\n\n" +
+                    $"This is usually caused by a missing Visual C++ Redistributable (x64) " +
+                    $"or an OS compatibility issue with the SkiaSharp rendering library.\n\n" +
+                    $"Download the latest VC++ Redistributable from:\n" +
+                    $"https://aka.ms/vs/17/release/vc_redist.x64.exe\n\n" +
+                    $"Error: {inner}",
+                    "Chart Initialization Error",
+                    System.Windows.MessageBoxButton.OK,
+                    System.Windows.MessageBoxImage.Error);
+                return;
+            }
             serverTab.AlertAcknowledged += (_, _) =>
             {
                 _emailAlertService.HideAllAlerts(8760, server.DisplayName);

Lite/Services/DeltaCalculator.cs

@@ -24,9 +24,9 @@ namespace PerformanceMonitorLite.Services;
 public class DeltaCalculator
 {
     /// <summary>
-    /// Cache structure: serverId -> collectorName -> key -> previousValue
+    /// Cache structure: serverId -> collectorName -> key -> (previousValue, timestamp)
     /// </summary>
-    private readonly ConcurrentDictionary<int, ConcurrentDictionary<string, ConcurrentDictionary<string, long>>> _cache = new();
+    private readonly ConcurrentDictionary<int, ConcurrentDictionary<string, ConcurrentDictionary<string, (long Value, DateTime? Timestamp)>>> _cache = new();
 
     private readonly ILogger? _logger;
 
@@ -63,12 +63,15 @@ public async Task SeedFromDatabaseAsync(DuckDbInitializer duckDb)
     /// Calculates the delta between the current value and the previous cached value.
     /// First-ever sighting (no baseline): returns currentValue so single-execution queries appear.
     /// Counter reset (value decreased): returns 0 to avoid inflated deltas from plan cache churn.
+    /// Gap detection: if collectionTime and maxGapSeconds are provided and the gap since the
+    /// last cached value exceeds maxGapSeconds, returns 0 to avoid inflated deltas after restarts.
     /// Thread-safe via atomic AddOrUpdate.
     /// </summary>
-    public long CalculateDelta(int serverId, string collectorName, string key, long currentValue, bool baselineOnly = false)
+    public long CalculateDelta(int serverId, string collectorName, string key, long currentValue,
+        bool baselineOnly = false, DateTime? collectionTime = null, int maxGapSeconds = 0)
     {
-        var serverCache = _cache.GetOrAdd(serverId, _ => new ConcurrentDictionary<string, ConcurrentDictionary<string, long>>());
-        var collectorCache = serverCache.GetOrAdd(collectorName, _ => new ConcurrentDictionary<string, long>());
+        var serverCache = _cache.GetOrAdd(serverId, _ => new ConcurrentDictionary<string, ConcurrentDictionary<string, (long Value, DateTime? Timestamp)>>());
+        var collectorCache = serverCache.GetOrAdd(collectorName, _ => new ConcurrentDictionary<string, (long Value, DateTime? Timestamp)>());
 
         long delta = 0;
 
@@ -80,15 +83,24 @@ public long CalculateDelta(int serverId, string collectorName, string key, long
             _ =>
             {
                 delta = baselineOnly ? 0 : currentValue;
-                return currentValue;
+                return (currentValue, collectionTime);
             },
             /* Update: compute delta atomically */
-            (_, previousValue) =>
+            (_, previous) =>
             {
-                delta = currentValue < previousValue
+                /* Gap detection: if too much time has passed since the last cached value,
+                   treat this as a new baseline to avoid inflated deltas after app restarts */
+                if (maxGapSeconds > 0 && collectionTime.HasValue && previous.Timestamp.HasValue
+                    && (collectionTime.Value - previous.Timestamp.Value).TotalSeconds > maxGapSeconds)
+                {
+                    delta = 0;
+                    return (currentValue, collectionTime);
+                }
+
+                delta = currentValue < previous.Value
                     ? 0              /* counter reset (plan cache eviction/re-entry) — not real new work */
-                    : currentValue - previousValue;
-                return currentValue;
+                    : currentValue - previous.Value;
+                return (currentValue, collectionTime);
             });
 
         return delta;
@@ -97,18 +109,18 @@ public long CalculateDelta(int serverId, string collectorName, string key, long
     /// <summary>
     /// Seeds a single value into the cache without computing a delta.
     /// </summary>
-    private void Seed(int serverId, string collectorName, string key, long value)
+    private void Seed(int serverId, string collectorName, string key, long value, DateTime? timestamp = null)
     {
-        var serverCache = _cache.GetOrAdd(serverId, _ => new ConcurrentDictionary<string, ConcurrentDictionary<string, long>>());
-        var collectorCache = serverCache.GetOrAdd(collectorName, _ => new ConcurrentDictionary<string, long>());
-        collectorCache[key] = value;
+        var serverCache = _cache.GetOrAdd(serverId, _ => new ConcurrentDictionary<string, ConcurrentDictionary<string, (long Value, DateTime? Timestamp)>>());
+        var collectorCache = serverCache.GetOrAdd(collectorName, _ => new ConcurrentDictionary<string, (long Value, DateTime? Timestamp)>());
+        collectorCache[key] = (value, timestamp);
     }
 
     private async Task SeedWaitStatsAsync(DuckDBConnection connection)
     {
         using var cmd = connection.CreateCommand();
         cmd.CommandText = @"
-SELECT server_id, wait_type, waiting_tasks_count, wait_time_ms, signal_wait_time_ms
+SELECT server_id, wait_type, waiting_tasks_count, wait_time_ms, signal_wait_time_ms, collection_time
 FROM wait_stats
 WHERE (server_id, collection_time) IN (
     SELECT server_id, MAX(collection_time) FROM wait_stats GROUP BY server_id
@@ -119,9 +131,10 @@ FROM wait_stats
         {
             var serverId = reader.GetInt32(0);
             var waitType = reader.GetString(1);
-            Seed(serverId, "wait_stats_tasks", waitType, reader.GetInt64(2));
-            Seed(serverId, "wait_stats_time", waitType, reader.GetInt64(3));
-            Seed(serverId, "wait_stats_signal", waitType, reader.GetInt64(4));
+            var ts = reader.IsDBNull(5) ? (DateTime?)null : reader.GetDateTime(5);
+            Seed(serverId, "wait_stats_tasks", waitType, reader.GetInt64(2), ts);
+            Seed(serverId, "wait_stats_time", waitType, reader.GetInt64(3), ts);
+            Seed(serverId, "wait_stats_signal", waitType, reader.GetInt64(4), ts);
             count++;
         }
         if (count > 0) _logger?.LogDebug("Seeded {Count} wait_stats baseline rows", count);
@@ -134,7 +147,8 @@ private async Task SeedFileIoStatsAsync(DuckDBConnection connection)
 SELECT server_id, database_name, file_name,
        num_of_reads, num_of_writes, read_bytes, write_bytes,
        io_stall_read_ms, io_stall_write_ms,
-       io_stall_queued_read_ms, io_stall_queued_write_ms
+       io_stall_queued_read_ms, io_stall_queued_write_ms,
+       collection_time
 FROM file_io_stats
 WHERE (server_id, collection_time) IN (
     SELECT server_id, MAX(collection_time) FROM file_io_stats GROUP BY server_id
@@ -147,14 +161,15 @@ FROM file_io_stats
             var dbName = reader.IsDBNull(1) ? "" : reader.GetString(1);
             var fileName = reader.IsDBNull(2) ? "" : reader.GetString(2);
             var deltaKey = $"{dbName}|{fileName}";
-            Seed(serverId, "file_io_reads", deltaKey, reader.IsDBNull(3) ? 0 : reader.GetInt64(3));
-            Seed(serverId, "file_io_writes", deltaKey, reader.IsDBNull(4) ? 0 : reader.GetInt64(4));
-            Seed(serverId, "file_io_read_bytes", deltaKey, reader.IsDBNull(5) ? 0 : reader.GetInt64(5));
-            Seed(serverId, "file_io_write_bytes", deltaKey, reader.IsDBNull(6) ? 0 : reader.GetInt64(6));
-            Seed(serverId, "file_io_stall_read", deltaKey, reader.IsDBNull(7) ? 0 : reader.GetInt64(7));
-            Seed(serverId, "file_io_stall_write", deltaKey, reader.IsDBNull(8) ? 0 : reader.GetInt64(8));
-            Seed(serverId, "file_io_stall_queued_read", deltaKey, reader.IsDBNull(9) ? 0 : reader.GetInt64(9));
-            Seed(serverId, "file_io_stall_queued_write", deltaKey, reader.IsDBNull(10) ? 0 : reader.GetInt64(10));
+            var ts = reader.IsDBNull(11) ? (DateTime?)null : reader.GetDateTime(11);
+            Seed(serverId, "file_io_reads", deltaKey, reader.IsDBNull(3) ? 0 : reader.GetInt64(3), ts);
+            Seed(serverId, "file_io_writes", deltaKey, reader.IsDBNull(4) ? 0 : reader.GetInt64(4), ts);
+            Seed(serverId, "file_io_read_bytes", deltaKey, reader.IsDBNull(5) ? 0 : reader.GetInt64(5), ts);
+            Seed(serverId, "file_io_write_bytes", deltaKey, reader.IsDBNull(6) ? 0 : reader.GetInt64(6), ts);
+            Seed(serverId, "file_io_stall_read", deltaKey, reader.IsDBNull(7) ? 0 : reader.GetInt64(7), ts);
+            Seed(serverId, "file_io_stall_write", deltaKey, reader.IsDBNull(8) ? 0 : reader.GetInt64(8), ts);
+            Seed(serverId, "file_io_stall_queued_read", deltaKey, reader.IsDBNull(9) ? 0 : reader.GetInt64(9), ts);
+            Seed(serverId, "file_io_stall_queued_write", deltaKey, reader.IsDBNull(10) ? 0 : reader.GetInt64(10), ts);
             count++;
         }
         if (count > 0) _logger?.LogDebug("Seeded {Count} file_io_stats baseline rows", count);
@@ -164,7 +179,7 @@ private async Task SeedPerfmonStatsAsync(DuckDBConnection connection)
     {
         using var cmd = connection.CreateCommand();
         cmd.CommandText = @"
-SELECT server_id, object_name, counter_name, instance_name, cntr_value
+SELECT server_id, object_name, counter_name, instance_name, cntr_value, collection_time
 FROM perfmon_stats
 WHERE (server_id, collection_time) IN (
     SELECT server_id, MAX(collection_time) FROM perfmon_stats GROUP BY server_id
@@ -177,7 +192,8 @@ FROM perfmon_stats
             var objectName = reader.IsDBNull(1) ? "" : reader.GetString(1);
             var counter = reader.IsDBNull(2) ? "" : reader.GetString(2);
             var instance = reader.IsDBNull(3) ? "" : reader.GetString(3);
-            Seed(serverId, "perfmon", $"{objectName}|{counter}|{instance}", reader.GetInt64(4));
+            var ts = reader.IsDBNull(5) ? (DateTime?)null : reader.GetDateTime(5);
+            Seed(serverId, "perfmon", $"{objectName}|{counter}|{instance}", reader.GetInt64(4), ts);
             count++;
         }
         if (count > 0) _logger?.LogDebug("Seeded {Count} perfmon_stats baseline rows", count);
@@ -202,8 +218,8 @@ FROM memory_grant_stats
                 var poolId = reader.IsDBNull(1) ? 0 : reader.GetInt32(1);
                 var semaphoreId = reader.IsDBNull(2) ? (short)0 : reader.GetInt16(2);
                 var deltaKey = $"{poolId}_{semaphoreId}";
-                Seed(serverId, "memory_grants_timeouts", deltaKey, reader.IsDBNull(3) ? 0 : reader.GetInt64(3));
-                Seed(serverId, "memory_grants_forced", deltaKey, reader.IsDBNull(4) ? 0 : reader.GetInt64(4));
+                Seed(serverId, "memory_grants_timeouts", deltaKey, reader.IsDBNull(3) ? 0 : reader.GetInt64(3), null);
+                Seed(serverId, "memory_grants_forced", deltaKey, reader.IsDBNull(4) ? 0 : reader.GetInt64(4), null);
                 count++;
             }
             if (count > 0) _logger?.LogDebug("Seeded {Count} memory_grant_stats baseline rows", count);

Lite/Services/LocalDataService.Perfmon.cs

@@ -95,13 +95,14 @@ public async Task<List<PerfmonTrendPoint>> GetPerfmonTrendAsync(int serverId, st
         command.CommandText = @"
 SELECT
     collection_time,
-    cntr_value,
-    delta_cntr_value
+    SUM(cntr_value) AS cntr_value,
+    SUM(delta_cntr_value) AS delta_cntr_value
 FROM v_perfmon_stats
 WHERE server_id = $1
 AND   counter_name = $2
 AND   collection_time >= $3
 AND   collection_time <= $4
+GROUP BY collection_time
 ORDER BY collection_time";
 
         command.Parameters.Add(new DuckDBParameter { Value = serverId });

Lite/Services/RemoteCollectorService.Perfmon.cs

@@ -178,9 +178,11 @@ WHERE pc.counter_name IN (
                     var instanceName = reader.IsDBNull(2) ? "" : reader.GetString(2);
                     var cntrValue = reader.GetInt64(3);
 
-                    /* Delta for per-second counters */
+                    /* Delta for per-second counters — gap detection at 5min (5x the 1-min collection interval)
+                       prevents inflated deltas after app restarts */
                     var deltaKey = $"{objectName}|{counterName}|{instanceName}";
-                    var deltaCntrValue = _deltaCalculator.CalculateDelta(serverId, "perfmon", deltaKey, cntrValue, baselineOnly: true);
+                    var deltaCntrValue = _deltaCalculator.CalculateDelta(serverId, "perfmon", deltaKey, cntrValue,
+                        baselineOnly: true, collectionTime: collectionTime, maxGapSeconds: 300);
 
                     var row = appender.CreateRow();
                     row.AppendValue(GenerateCollectionId())
@@ -192,7 +194,7 @@ WHERE pc.counter_name IN (
                        .AppendValue(instanceName)
                        .AppendValue(cntrValue)
                        .AppendValue(deltaCntrValue)
-                       .AppendValue(600) /* 10-minute interval */
+                       .AppendValue(60) /* 1-minute collection interval */
                        .EndRow();
 
                     rowsCollected++;