def __init__(self, package, version):
        """Construct a new requirement.

        :param package: the package name.
        :param version: a semver compatible version specification.
        """
        self.package = package
        self.version = version
        if self.version and not re.match(r"[<=>~]", version[:2]):
            self.version = "=={}".format(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)