# paste this code at the end of VectorFieldPlot 2.5
# https://commons.wikimedia.org/wiki/User:Geek3/VectorFieldPlot
doc = FieldplotDocument('VFPt_cylindermagnet_field-representations', commons=True,
    width=600, height=600)

R = 0.6
Lhalf = 1.0
field = Field([['coil', {'x':0, 'y':0, 'phi':pi/2, 'R':R, 'Lhalf':Lhalf, 'I':1}]])
doc.draw_magnets(field)

n = 26
for i in range(n):
    a = R * (-1 + 2. * (0.5 + i) / n)
    line = FieldLine(field, [a, 0.], directions='both', maxr=5.0)
    def arrow_pot(xy):
        return (((xy[0]/(2.5*R))**2+(((xy[1]-Lhalf)/(2*R))**2)-1) *
            ((xy[0]/(2.5*R))**2+(((xy[1]+Lhalf)/(2*R))**2)-1))
    if i in [0, n-1]:
        arrow_pot = lambda xy: xy[1]
    doc.draw_line(line, arrows_style={'scale':1.3,
        'at_potentials':[0.], 'potential':arrow_pot,
        'condition_func':lambda xy: fabs(xy[0])>R or fabs(xy[1])>Lhalf})

doc.write()