  lines, labels = rgrids()
     lines, labels = rgrids(radii, labels=None, angle=22.5, fmt=None, **kwargs)

    When called with no arguments, `.rgrids` simply returns the tuple
    (*lines*, *labels*). When called with arguments, the labels will
    appear at the specified radial distances and angle.

    Parameters
    ----------
    radii : tuple with floats
        The radii for the radial gridlines

    labels : tuple with strings or None
        The labels to use at each radial gridline. The
        `matplotlib.ticker.ScalarFormatter` will be used if None.

    angle : float
        The angular position of the radius labels in degrees.

    fmt : str or None
        Format string used in `matplotlib.ticker.FormatStrFormatter`.
        For example '%f'.

    Returns
    -------
    lines : list of `.lines.Line2D`
        The radial gridlines.

    labels : list of `.text.Text`
        The tick labels.

    Other Parameters
    ----------------
    **kwargs
        *kwargs* are optional `.Text` properties for the labels.

    See Also
    --------
    .pyplot.thetagrids
    .projections.polar.PolarAxes.set_rgrids
    .Axis.get_gridlines
    .Axis.get_ticklabels

    Examples
    --------
    ::

      # set the locations of the radial gridlines
      lines, labels = rgrids( (0.25, 0.5, 1.0) )

      # set the locations and labels of the radial gridlines
      lines, labels = rgrids( (0.25, 0.5, 1.0), ('Tom', 'Dick', 'Harry' ))
    z"rgrids only defined for polar axesc