show better error message, when no data points are found

pdb-plotting/src/main/java/org/lucares/recommind/logs/InternalPlottingException.java

@@ -4,6 +4,10 @@ public class InternalPlottingException extends Exception {
 
 	private static final long serialVersionUID = 1L;
 
+	public InternalPlottingException() {
+		super();
+	}
+
 	public InternalPlottingException(final String message, final Throwable cause) {
 		super(message, cause);
 	}

pdb-plotting/src/main/java/org/lucares/recommind/logs/NoDataPointsException.java

@@ -0,0 +1,10 @@
+package org.lucares.recommind.logs;
+
+public class NoDataPointsException extends InternalPlottingException {
+
+	private static final long serialVersionUID = 1054594230615520105L;
+
+	public NoDataPointsException() {
+		super();
+	}
+}

pdb-plotting/src/main/java/org/lucares/recommind/logs/Plotter.java

@@ -10,7 +10,9 @@ import java.nio.file.Files;
 import java.nio.file.LinkOption;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.time.Instant;
 import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
 import java.time.format.DateTimeFormatter;
 import java.time.temporal.ChronoUnit;
 import java.util.ArrayList;
@@ -74,8 +76,8 @@ public class Plotter {
 
 			final Result result = db.get(query, groupBy);
 
-			OffsetDateTime maxDate = OffsetDateTime.MIN;
-			OffsetDateTime minDate = OffsetDateTime.MAX;
+			OffsetDateTime maxDate = OffsetDateTime.ofInstant(Instant.ofEpochMilli(Long.MIN_VALUE), ZoneOffset.UTC);
+			OffsetDateTime minDate = OffsetDateTime.ofInstant(Instant.ofEpochMilli(Long.MAX_VALUE), ZoneOffset.UTC);
 
 			for (final GroupResult groupResult : result.getGroups()) {
 
@@ -94,6 +96,10 @@ public class Plotter {
 				}
 			}
 
+			if (dataSeries.isEmpty()) {
+				throw new NoDataPointsException();
+			}
+
 			sortAndLimit(dataSeries, plotSettings);
 
 			final File outputFile = File.createTempFile("out", ".png", outputDir.toFile());