def _generateSpecialOrthoOptsCentre(displayCtx, xax, yax, canvas):
    """Used by the generation functions for the ``xcentre``, ``ycentre``,
    and ``zcentre`` options.
    """
    x, y = canvas.getDisplayCentre()
    xlo = displayCtx.bounds.getLo(xax)
    ylo = displayCtx.bounds.getLo(yax)
    xlen = displayCtx.bounds.getLen(xax)
    ylen = displayCtx.bounds.getLen(yax)
    x = -1 + 2 * (x - xlo) / xlen
    y = -1 + 2 * (y - ylo) / ylen
    return ['{: 0.5f}'.format(x), '{: 0.5f}'.format(y)]
