def __repr__(self):
        return "<Requirement(package={package}, version='{version}')>".format(
            package=self.package, version=self.version)
def winding_flux(pos, flux, ax=0):
    """plot flux vs position"""
    if ax == 0:
        ax = plt.gca()
    ax.set_title('Winding Flux / Vs')
    ax.grid(True)
    for p, f in zip(pos, flux):
        ax.plot(p, f)