diff --git a/pdb-plotting/src/main/java/org/lucares/pdb/plot/api/ScatterAggregator.java b/pdb-plotting/src/main/java/org/lucares/pdb/plot/api/ScatterAggregator.java
index ff9d266..6972366 100644
--- a/pdb-plotting/src/main/java/org/lucares/pdb/plot/api/ScatterAggregator.java
+++ b/pdb-plotting/src/main/java/org/lucares/pdb/plot/api/ScatterAggregator.java
@@ -56,7 +56,9 @@ public class ScatterAggregator implements CustomAggregator {
     public void addValue(final long epochMilli, final long value) {
         final long roundedEpochMilli = epochMilli - epochMilli % epochMillisPerPixel;
         final long roundedValue = value - value % valuesPerPixel;
-        matrix2d.put(roundedEpochMilli, roundedValue, 1);
+        if (roundedEpochMilli > 0 && roundedValue >= 0) {
+            matrix2d.put(roundedEpochMilli, roundedValue, 1);
+        }
     }
 
     @Override