Google Maps v3 setting a circle with editable radius but not center

I have managed to put a circle on a map and make it editable:

var circle = new google.maps.Circle({
          map: map,
          radius: 1609.344, // 1 mile
          editable: true,
          fillOpacity: 0.25
        });


circle.bindTo('center', marker, 'position');

However, this makes both the radius, and center editable. I only want the radius to be editable. I can't see a way to have that level of control via google.maps.Circle.

What I am after, is pretty much what is here:

http://www.freemaptools.com/radius-around-point.htm

However I can't see how it is done as their code looks obfuscated.