def save(self, doc):
        if doc.id:
            return self.update(doc)
        else:
            return self.insert(doc)
def voltage(title, pos, voltage, ax=0):
    """plot voltage vs. position"""
    if ax == 0:
        ax = plt.gca()
    ax.set_title('{} / V'.format(title))
    ax.grid(True)
    ax.plot(pos, voltage)