group by multiple fields

Before we could only group by a single field. But it is acutally
very useful to group by multiple fields. For example to see the
graph for a small set of methods grouped by host and project.

pdb-api/src/main/java/org/lucares/pdb/api/Tags.java

@@ -1,7 +1,8 @@
 package org.lucares.pdb.api;
 
 import java.util.Collections;
-import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
@@ -29,16 +30,25 @@ public class Tags {
 		return EMPTY;
 	}
 
+	public static Tags create(final String key, final String value) {
+		final Map<String, Tag> tags = new LinkedHashMap<>(1);
+		tags.put(key, new Tag(key, value));
+		return new Tags(tags);
+	}
+
 	public static Tags create(final String key1, final String value1, final String key2, final String value2) {
-		final Map<String, Tag> tags = new HashMap<>(2);
+		final Map<String, Tag> tags = new LinkedHashMap<>(2);
 		tags.put(key1, new Tag(key1, value1));
 		tags.put(key2, new Tag(key2, value2));
 		return new Tags(tags);
 	}
 
-	public static Tags create(final String key, final String value) {
-		final Map<String, Tag> tags = new HashMap<>(1);
-		tags.put(key, new Tag(key, value));
+	public static Tags create(final String key1, final String value1, final String key2, final String value2,
+			final String key3, final String value3) {
+		final Map<String, Tag> tags = new LinkedHashMap<>(3);
+		tags.put(key1, new Tag(key1, value1));
+		tags.put(key2, new Tag(key2, value2));
+		tags.put(key3, new Tag(key3, value3));
 		return new Tags(tags);
 	}
 
@@ -46,7 +56,7 @@ public class Tags {
 		Objects.requireNonNull(key, "key must not be null");
 		Objects.requireNonNull(value, "value must not be null");
 
-		final Map<String, Tag> newTags = new HashMap<>(tags);
+		final Map<String, Tag> newTags = new LinkedHashMap<>(tags);
 
 		newTags.put(key, new Tag(key, value));
 
@@ -82,7 +92,7 @@ public class Tags {
 
 	@Override
 	public String toString() {
-		return String.valueOf(tags);
+		return String.valueOf(tags.values());
 	}
 
 	@Override
@@ -142,4 +152,23 @@ public class Tags {
 		return s.substring(0, Math.min(maxLength, s.length()));
 	}
 
+	public Tags subset(final List<String> groupByFields) {
+
+		Tags result = new Tags();
+
+		for (final String field : groupByFields) {
+			final String value = getValue(field);
+
+			if (value != null) {
+				result = result.copyAdd(field, value);
+			}
+		}
+
+		return result;
+	}
+
+	public boolean isEmpty() {
+		return tags.isEmpty();
+	}
+
 }