added import/export menu

Signed-off-by: Sven Nierlein <sven@nierlein.de>
smarted aggregation with undefined values

Signed-off-by: Sven Nierlein <sven@nierlein.de>
made chart legend draggable
make dragged legend permanent

Signed-off-by: Sven Nierlein <sven@nierlein.de>
make dragging smoother