Create a Create a ``DataSpec`` dict to generate a ``CumSum`` expression
    for a ``ColumnDataSource``.

    Examples:

        .. code-block:: python

            p.wedge(start_angle=cumsum('angle', include_zero=True),
                    end_angle=cumsum('angle'),
                    ...)

        will generate a ``CumSum`` expressions that sum the ``"angle"`` column
        of a data source. For the ``start_angle`` value, the cumulative sums
        will start with a zero value. For ``start_angle``, no initial zero will
        be added (i.e. the sums will start with the first angle value, and
        include the last).

def cumsum(field, include_zero=False):
    ''' Create a Create a ``DataSpec`` dict to generate a ``CumSum`` expression
    for a ``ColumnDataSource``.

    Examples:

        .. code-block:: python

            p.wedge(start_angle=cumsum('angle', include_zero=True),
                    end_angle=cumsum('angle'),
                    ...)

        will generate a ``CumSum`` expressions that sum the ``"angle"`` column
        of a data source. For the ``start_angle`` value, the cumulative sums
        will start with a zero value. For ``start_angle``, no initial zero will
        be added (i.e. the sums will start with the first angle value, and
        include the last).

    '''
    return expr(CumSum(field=field, include_zero=include_zero))