code: public final String toSvgStr(String units) {
          if (!units.equals(UNITS_MM) && !units.equals(UNITS_PX))
              throw new IllegalArgumentException("Units must be 'px' or 'mm'!");
          return toVecStr(SVG_FMT, units);
      }
docs: Render the image to an SVG image.
  
  @param units the units for SVG - 'px' or 'mm'
  @return svg XML content