# paste this code at the end of VectorFieldPlot 3.1
# https://commons.wikimedia.org/wiki/User:Geek3/VectorFieldPlot
doc = FieldplotDocument('VFPt_minus_thumb_potential+contour',
    width=180, height=180, unit=30, commons=True)
field = Field([ ['monopole', {'x':0, 'y':0, 'Q':-1}] ])
doc.draw_charges(field)
nlines = 16
for iline in range(nlines):
    a = 2.0 * pi * (0.5 + iline) / nlines
    line = FieldLine(field, [0, 0], start_v=[cos(a), sin(a)],
        directions='backward', maxr=5)
    doc.draw_line(line, linewidth=1.2, arrows_style={'dist':2, 'scale':1.8,
        'max_arrows':1, 'offsets':{'end':1.05}, 'fixed_ends':{'end':True} })

U0 = fabs(field.V(sc.array([15./30., 0.])))
U1 = field.V(sc.array([14.65/30., 0.]))
doc.draw_scalar_field(func=field.V, cmap=doc.cmap_AqYlFs, vmin=-U0, vmax=U0)
doc.draw_contours(func=field.V, linewidth=0.7, linecolor='#444444',
    levels=sc.linspace(U1, 0, 12))
doc.write()