File:VFPt four charges.svg

# paste this code at the end of VectorFieldPlot 1.0
doc = FieldplotDocument('VFPt_four_charges', width=600, height=600, commons=True)
field = Field({'monopoles':[[-1,0,-1], [0,1,1], [1,0,-1], [0,-1,1]]})
doc.draw_charges(field)
n = 8
Fsum = ig.quad(lambda t: sc.dot(field.F([.5*sin(t), 1-0.5*cos(t)]),
    [sin(t), -cos(t)]), 0, pi)[0]
for i in range(n):
    a = (i + 0.5) / n
    w = op.brentq(lambda w: ig.quad(lambda t: sc.dot(field.F([0.5*sin(t),
    	1-0.5*cos(t)]), [sin(t), -cos(t)]), 0, w)[0] - a * Fsum, 0, pi)
    for w1 in range(4):
    	line = FieldLine(field, rot([.5*sin(w), 1.0-0.5*cos(w)], w1 * pi/2),
    	    directions='both')
    	doc.draw_line(line, arrows_style={'dist':1.5})
doc.write()