# paste this code at the end of VectorFieldPlot 1.7
doc = FieldplotDocument('VFPt_charges_plus_plus2', commons=True)
x0, y0 = -1., 0.
x1, y1 = 1., 0.
field = Field({'monopoles':[[x0, y0, 1], [x1, y1, 1]]})
doc.draw_charges(field)

def startpath(t):
    phi = 2 * pi * t
    return sc.array([x0 + 1 * cos(phi), y0 + 1 * sin(phi)])
def dstartpath(t):
    return (startpath(t+1e-6) - startpath(t-1e-6)) / 2e-6
def FieldSum(t0, t1):
    return ig.quad(lambda t:
        sc.cross(field.F(startpath(t)), dstartpath(t)), t0, t1)[0]
Ftotal = FieldSum(0, 1)
def startpos(s):
    t = op.brentq(lambda t: FieldSum(0, t) / Ftotal - s, 0, 1)
    return startpath(t)

n = 16
for i in range(n):
    p0 = startpos((0.5 + i) / n)
    for p0 in [p0, [-p0[0], p0[1]]]:
        line = FieldLine(field, p0, directions='both', maxr=3)
        doc.draw_line(line, arrows_style={'dist':3.0, 'min_arrows':1,
            'max_arrows':1, 'fixed_ends':[True, False, False, False]})
doc.write()