Experiment crashes when plotting stats from the same database

[danijar]

  File "/home/dhafner/repository/mindpark/mindpark/core/metric.py", line 31, in __call__
    self.flush()
  File "/home/dhafner/repository/mindpark/mindpark/core/metric.py", line 39, in flush
    connection.execute(insert, self._buffer)
...
  File "/home/dhafner/.local/lib/python3.4/site-packages/sqlalchemy/engine/default.py", line 423, in do_commit
    dbapi_connection.commit()
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) database is locked