class_docstr: Preallocation strategies for rolling window over expanding data set
  
  Restrictions: major_axis can only be a DatetimeIndex for now
class_docstr_tok: ["Preallocation","strategies","for","rolling","window","over","expanding","data","set","Restrictions",":","major_axis","can","only","be","a","DatetimeIndex","for","now"]
docstr: Roll window worth of data up to position zero.
  Save the effort of having to expensively roll at each iteration
docstr_tok: ["Roll","window","worth","of","data","up","to","position","zero",".","Save","the","effort","of","having","to","expensively","roll","at","each","iteration"]
code: def _roll_data(self):
      """
      Roll window worth of data up to position zero.
      Save the effort of having to expensively roll at each iteration
      """
  
      self.buffer.values[:, : self._window, :] = self.buffer.values[
          :, -self._window :, :
      ]
      self.date_buf[: self._window] = self.date_buf[-self._window :]
      self._pos = self._window
  
imports: import datetime
  from copy import deepcopy
  import numpy
  import pandas
label_desc: Use image node_id 14 for calling the RollingPanel obj's underlying member method code with example usage: obj._roll_data() without return types