Set the r attribute of the subelement relatively to the circumscribed size (length of the diagonal of the bounding box) of the element referenced by the selector in the ref attribute. If the value is in the [0, 1] interval (or expressed in percentages, e.g. '80%'), the r of the subelement will be set as a percentage of the size of the referenced element. If the value is <0 or >1, the size of the subelement will be smaller/bigger than the size of the referenced element by the amount specified. Note that this makes sense only for SVG elements that support r attribute, such as <circle>.