def plot_xtb(fignum, XTB, Bs, e, f):
    """ function to plot series of chi measurements as a function of temperature, holding frequency constant and varying B
    """
    plt.figure(num=fignum)
    plt.xlabel('Temperature (K)')
    plt.ylabel('Susceptibility (m^3/kg)')
    k = 0
    Blab = []
    for field in XTB:
        T, X = [], []
        for xt in field:
            X.append(xt[0])
            T.append(xt[1])
        plt.plot(T, X)
        plt.text(T[-1], X[-1], '%8.2e' % (Bs[k]) + ' T')
#        Blab.append('%8.1e'%(Bs[k])+' T')
        k += 1
    plt.title(e + ': f = ' + '%i' % (int(f)) + ' Hz')
function to plot series of chi measurements as a function of temperature, holding frequency constant and varying B