code: function makePlusMinus(formatter) {
    return function plusMinusWrapped(value) {
      if (value != null && value > 0) {
        return '+' + formatter(value);
      }
  
      return formatter(value);
    };
  }
docs: Wraps a formatting function and puts a + in front of formatted value
  if it is positive.
  
  @param {Function} formatter formatter function
  @return {Function} a formatter function