Set cx attribute of the subelement relatively to the width of the element referenced to by the selector in ref attribute. If the value is in the [0, 1] interval (or expressed in percentages, e.g. '80%'), the cx of the subelement will be set as a percentage of the width of the referenced element. If the value is <0 or >1, the height of the subelement will be smaller/bigger than the width of the referenced element by the amount specified. Note that this makes sense only for SVG elements that support rx and ry attributes, such as <ellipse>.