fixing sorting

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

src/main/java/de/srsoftware/widerhall/data/Post.java

@@ -18,6 +18,7 @@ import java.sql.Timestamp;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.TreeMap;
 
 import static de.srsoftware.widerhall.Constants.*;
 import static de.srsoftware.widerhall.Constants.VARCHAR;
@@ -173,9 +174,9 @@ public class Post {
         return this;
     }
 
-    public static HashMap<String, Object> summarize(MailingList list) throws SQLException {
+    public static Map<String, Object> summarize(MailingList list) throws SQLException {
         var sql = new StringBuilder("SELECT count(*) as count,strftime('%Y-%m',date/1000,'unixepoch') as month FROM Posts WHERE list = ? GROUP BY month ORDER BY month;");
-        var map = new HashMap<String,Object>();
+        var map = new TreeMap<String,Object>();
         var rs = Database.open().query(sql).compile(list.email()).exec();
         while (rs.next()) map.put(rs.getString("month"),rs.getInt("count"));
         rs.close();