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-ui/src/main/java/org/lucares/pdbui/domain/PlotRequest.java

@@ -1,5 +1,7 @@
 package org.lucares.pdbui.domain;
 
+import java.util.List;
+
 public class PlotRequest {
 	private String query;
 
@@ -7,7 +9,7 @@ public class PlotRequest {
 
 	private int width = 1000;
 
-	private String groupBy;
+	private List<String> groupBy;
 
 	private LimitBy limitBy = LimitBy.NO_LIMIT;
 
@@ -46,11 +48,11 @@ public class PlotRequest {
 		return query + ":" + height + "x" + width;
 	}
 
-	public String getGroupBy() {
+	public List<String> getGroupBy() {
 		return groupBy;
 	}
 
-	public void setGroupBy(final String groupBy) {
+	public void setGroupBy(final List<String> groupBy) {
 		this.groupBy = groupBy;
 	}