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

@@ -36,6 +36,8 @@ public class Plotter {
 	private static final Logger LOGGER = LoggerFactory.getLogger(Plotter.class);
 	private static final Logger METRICS_LOGGER = LoggerFactory.getLogger("org.lucares.metrics.plotter");
 
+	private static final String DEFAULT_GROUP = "<none>";
+
 	private final PerformanceDb db;
 	private final Path tmpBaseDir;
 	private final Path outputDir;
@@ -68,7 +70,7 @@ public class Plotter {
 			final List<DataSeries> dataSeries = new ArrayList<>();
 
 			final String query = plotSettings.getQuery();
-			final String groupBy = plotSettings.getGroupBy();
+			final List<String> groupBy = plotSettings.getGroupBy();
 			final int height = plotSettings.getHeight();
 			final int width = plotSettings.getWidth();
 			final OffsetDateTime dateFrom = plotSettings.dateFrom();
@@ -165,12 +167,18 @@ public class Plotter {
 
 		final StringBuilder result = new StringBuilder();
 
-		assert tags.getKeys().size() <= 1;
-		tags.forEach((k, v) -> {
-			result.append(v);
-		});
+		if (tags.isEmpty()) {
+			result.append(DEFAULT_GROUP);
+		} else {
+			tags.forEach((k, v) -> {
+				if (result.length() > 0) {
+					result.append(" / ");
+				}
+				result.append(v);
+			});
+		}
 
-		result.append("(");
+		result.append(" (");
 		result.append(values);
 		result.append(")");