docs: private SVGPath append(double x) {
      if(!Double.isFinite(x)) {
        throw new IllegalArgumentException("Cannot draw an infinite/NaN position.");
      }
      if(x >= 0) {
        final int l = buf.length();
        if(l > 0) {
          char c = buf.charAt(l - 1);
          assert c != 'e' && c != 'E' : "Invalid exponential in path";
          if(c >= '0' && c <= '9')
            buf.append(' ');
        }
      }
      buf.append(SVGUtil.FMT.format(x));
      return this;
    }