Skip first reading in delta plugins as it is not very meaningful

src/includes/SensorBase.h

@@ -35,6 +35,7 @@ public:
 		_cacheIndex(0),
 		_cache(nullptr),
 		_delta(false),
+		_firstReading(true),
 		_readingQueue(nullptr),
 		_sinkFile(nullptr) {
 
@@ -58,6 +59,7 @@ public:
 		_cacheIndex(0),
 		_cache(nullptr),
 		_delta(other._delta),
+		_firstReading(true),
 		_latestValue(other._latestValue),
 		_lastRawValue(other._lastRawValue),
         _lastSentValue(other._lastSentValue),
@@ -78,6 +80,7 @@ public:
 		_cacheIndex = 0;
 		_cache.reset(nullptr);
 		_delta = other._delta;
+		_firstReading = true;
 		_latestValue.timestamp	= other._latestValue.timestamp;
 		_latestValue.value		= other._latestValue.value;
 		_lastRawValue.timestamp = other._lastRawValue.timestamp;
@@ -133,10 +136,16 @@ public:
 	void storeReading(reading_t rawReading, double factor=1.0, unsigned long long maxValue=ULLONG_MAX) {
 		reading_t reading = rawReading;
 		if( _delta ) {
-			if (rawReading.value < _lastRawValue.value)
-				reading.value = (rawReading.value + (maxValue - _lastRawValue.value)) * factor;
-			else
-				reading.value = (rawReading.value - _lastRawValue.value) * factor;
+		    if (!_firstReading) {
+              if (rawReading.value < _lastRawValue.value)
+                  reading.value = (rawReading.value + (maxValue - _lastRawValue.value)) * factor;
+              else
+                  reading.value = (rawReading.value - _lastRawValue.value) * factor;
+		    } else {
+		      _firstReading = false;
+		      _lastRawValue = rawReading;
+		      return;
+		    }
 			_lastRawValue = rawReading;
 		}
 		else
@@ -172,6 +181,7 @@ protected:
 	unsigned int _cacheIndex;
 	std::unique_ptr<reading_t[]> _cache;
 	bool _delta;
+	bool _firstReading;
 	reading_t _latestValue;
 	reading_t _lastRawValue;
 	reading_t _lastSentValue;