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>`.