See SVM's separating hyperplane for 1D, 2D, and 3D data.

import numpy as np
import matplotlib.pyplot as plt
from sklearn import svm, datasets
from io import StringIO # to treat a string as a file
import pandas as pd
from mpl_toolkits.mplot3d import Axes3D

Start with 1D toy data:

data_string = """
x1,  y
 0, -1
 1, -1
 3,  1
 4,  1
"""
df = pd.read_csv(StringIO(data_string), sep='\s*,\s+', engine='python')
X = df[['x1']] # a 2D DataFrame with one column; clf.predict(), below, wants 2D X
y = df.y
print(f'X={X}, y={y}')

X=   x1
0   0
1   1
2   3
3   4, y=0   -1
1   -1
2    1
3    1
Name: y, dtype: int64

clf = svm.SVC(kernel="linear", C=1000) # 'SVC' = 'support vector classification'
clf.fit(X, y) # X is 2D
print(f'clf.coef_={clf.coef_}')
print(f'clf.intercept_={clf.intercept_}')

clf.coef_=[[1.]]
clf.intercept_=[-2.]

plt.scatter(x=df.x1, y=np.zeros(shape=df.x1.shape), c=y, cmap=plt.cm.Paired) # plt.scatter() wants 1D column df.x1
plt.xlim(-1, 5)
plt.axhline(y=0, c='k')
ax = plt.gca()
ax.axes.get_yaxis().set_visible(False)
# The boundary is given by wx + b = 0 => x = -b / w.
boundary = -clf.intercept_[0] / clf.coef_[0]
plt.axvline(x=boundary, c='k', label=r'decision boundary $\mathbf{wx} + b = 0$')
plt.axvline(x=boundary+1, c='g', linestyle=':', label=r'+1 support $\mathbf{wx} + b =  1$')
plt.axvline(x=boundary-1, c='r', linestyle=':', label=r'-1 support $\mathbf{wx} + b = -1$')
plt.legend()
plt.show(block=False)

# make a few predictions
X_new = pd.DataFrame({'x1': [0, 1.5, 2, 2.5, 4]})
clf.predict(X_new)

array([-1, -1,  1,  1,  1])

Now try 2D toy data:

data_string = """
x1, x2,  y
 0,  0, -1
-1,  1, -1
 1, -1, -1
 0,  1,  1
 1,  1,  1
 1,  0,  1
"""
df = pd.read_csv(StringIO(data_string), sep='\s*,\s+', engine='python')
df # a data frame is like a spreadsheet

	x1	x2	y
0	0	0	-1
1	-1	1	-1
2	1	-1	-1
3	0	1	1
4	1	1	1
5	1	0	1

X = df[['x1', 'x2']]
y = df.y
print(f'X={X}, y={y}')

X=   x1  x2
0   0   0
1  -1   1
2   1  -1
3   0   1
4   1   1
5   1   0, y=0   -1
1   -1
2   -1
3    1
4    1
5    1
Name: y, dtype: int64

clf = svm.SVC(kernel="linear", C=1000)
clf.fit(X, y)
print(f'clf.coef_={clf.coef_}')
print(f'clf.intercept_={clf.intercept_}')

clf.coef_=[[2. 2.]]
clf.intercept_=[-1.]

The decision boundary is defined by $\mathbf{wx} + b = 0$. To plot it in the 2D $\mathbf{x}$ case, we use $x$-coordinate $x_1$ and $y$-coordinate $x_2$. Let's solve for the $y$-coordinate $x_2$: $\mathbf{wx} + b = 0$ $\implies w_1 x_1 + w_2 x_2 + b = 0 \implies x_2 = -\frac{w_1 x_1 + b}{w_2}$. Now in Python $\mathbf{w}$ is given by clf.coef_[0] and $b$ is given by clf.intercept_, so the boundary is

x2 = -(clf.coef_[0][0] * x1 + clf.intercept_) / clf.coef_[0][1].

The constraints are $\mathbf{wx} + b \ge 1$ if $y = +1$ and $\mathbf{wx} + b \le -1$ if $y = -1$; use $=$ instead of $\ge$ or $\le$ to get the margin edges.

With these formulas in mind, let's plot the data, decision boundary, and margin edges.

# Plot the data:
# First plot the y == -1 values red with x coordinate from the x1 column of X
# and y coordinate from the x2 column of X.
plt.plot(df.x1[y == -1], df.x2[y == -1], '.', color='red', label='y=-1 data')
# Second plot the y == 1 values blue.
plt.plot(df.x1[y ==  1], df.x2[y ==  1], '.', color='blue', label='y=1 data')
plt.axis('square')
plt.grid()
plt.xlim(-4, 4)
plt.ylim(-4, 4)

# add the decision boundary and margin boundaries
x1 = df.x1
x2 = -(clf.coef_[0][0] * x1 + clf.intercept_) / clf.coef_[0][1]
plt.plot(x1, x2, label=r'decision boundary $\mathbf{wx} + b = 0$')
plt.plot(x1, x2 + 1 / clf.coef_[0][1], linestyle=':', color='blue', label=r'+1 support $\mathbf{wx} + b =  1$')
plt.plot(x1, x2 - 1 / clf.coef_[0][1], linestyle=':', color='red', label=r'+1 support $\mathbf{wx} + b = -1$')
plt.plot([0, clf.coef_[0][0]], [0, clf.coef_[0][1]], label=r'normal') # arguments are x1, x2, y1, y2
plt.axvline(x=0, c='k')
plt.axhline(y=0, c='k')
plt.legend()
plt.show(block=False)

# make a couple of predictions
X_new = pd.DataFrame({'x1': [2, -2], 'x2': [2, -2]})
print(f'clf.predict({X_new})={clf.predict(X_new)}')

clf.predict(   x1  x2
0   2   2
1  -2  -2)=[ 1 -1]

Now let's try 3D data:

(modified from Support Vector Machines (SVM) clearly explained; data described at Iris plants dataset, which says "One class is linearly separable from the other 2; the latter are NOT linearly separable from each other," and sklearn.datasets.load_iris¶)

iris = datasets.load_iris()
# inspect the data:
print(f'iris.feature_names={iris.feature_names}')
print(f'first few rows:\n{iris.data[0:3, :]}')
print(f'first few target values: {iris.target[0:3]} (there are 0, 1, and 2 values)')

iris.feature_names=['sepal length (cm)', 'sepal width (cm)', 'petal length (cm)', 'petal width (cm)']
first few rows:
[[5.1 3.5 1.4 0.2]
 [4.9 3.  1.4 0.2]
 [4.7 3.2 1.3 0.2]]
first few target values: [0 0 0] (there are 0, 1, and 2 values)

X = iris.data[:, :3]  # we only take the first three features (columns) for 3D visualization
Y = iris.target
# make it a binary classification problem by excluding the Y==2 examples
X = X[np.logical_or(Y==0, Y==1)]
Y = Y[np.logical_or(Y==0, Y==1)]
print(f'X=\n{X[0:3]}..., Y={Y[:3]}...')

X=
[[5.1 3.5 1.4]
 [4.9 3.  1.4]
 [4.7 3.2 1.3]]..., Y=[0 0 0]...

model = svm.SVC(kernel='linear')
clf = model.fit(X, Y)
print(f'clf.coef_={clf.coef_}')
print(f'clf.intercept_={clf.intercept_}')

clf.coef_=[[ 0.30488777 -0.71125336  1.15277433]]
clf.intercept_=[-2.23510531]

# plot the data
fig = plt.figure()
ax  = fig.add_subplot(111, projection='3d')
ax.plot3D(X[Y==0,0], X[Y==0,1], X[Y==0,2],'ob')
ax.plot3D(X[Y==1,0], X[Y==1,1], X[Y==1,2],'sr')

# The equation of the separating plane is given by all x so that
# np.dot(svc.coef_[0], x) + b = 0. Solve for x_3, the third coordinate in
# (x_1, x_2, x_3) or, in plotting notation, (x, y, z).)
def z(x, y): # define a function that gives z from x and y based on the model
    return((-clf.intercept_[0] - clf.coef_[0][0]*x - clf.coef_[0][1]*y) /
           clf.coef_[0][2])
linspace = np.linspace(start=-5, stop=5, num=30) # 30 values between -5 and 5
x,y = np.meshgrid(linspace, linspace) # make 2D coordinate array from two 1D vectors

# https://matplotlib.org/2.0.2/mpl_toolkits/mplot3d/tutorial.html#surface-plots
ax.plot_surface(X=x, Y=y, Z=z(x,y))
ax.view_init(30, 60)
plt.show(block=False)