contour levels to span the data.

        The target number of levels, *N*, is used only when the
        scale is not log and default locator is used.

        We need two more levels for filled contours than for
        line contours, because for the latter we need to specify
        the lower and upper boundary of each range. For example,
        a single contour boundary, say at z = 0, requires only
        one contour line, but two filled regions, and therefore
        three levels to provide boundaries for both regions.
        Nr˜