Fret diagrams without 'x' and 'o'

I would like to propose a small improvement to the fret diagram feature: The ability to switch not only between 'x' and 'o', but also choose not to show anything above a given string.

I would suggest to implement this similarly to the switch between 'x' and 'o'. A third click would hide the sign while another click into the area would get back the 'x' or 'o'.