'Create and return a new GridlineCollection instance.
*which* : "major" or "minor"
*axis* : "both", "x" or "y"'

def new_gridlines(self, ax):

    gridlines = GridlinesCollection(None, transform=ax.transData, colors=rcParams[u'grid.color'], linestyles=rcParams[u'grid.linestyle'], linewidths=rcParams[u'grid.linewidth'])
    ax._set_artist_props(gridlines)
    gridlines.set_grid_helper(self)
    ax.axes._set_artist_props(gridlines)
    return gridlines
