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I am running logistic regression on iris dataset. I computed thetas and this is how I draw a decision boundary line.

x_values = ([min(X_train[:,0]), max(X_train[:,0])])
y_values = - (theta[0] + np.dot(theta[1], x_values)) / theta[2]
plt.scatter(X_train[:, 0], X_train[:, 1], c=y_train, cmap=plt.cm.Set1, edgecolor='k')
plt.plot(x_values, y_values )

enter image description here

I tried this, but the result is odd. 

X= np.c_[ X_train, np.zeros(100) ]   

theta = theta.reshape(3)

d=0

xx, yy = np.meshgrid(np.arange(np.min(X_reduced[:, 0]), np.max(X_reduced[:, 0])), np.arange(np.min(X_reduced[:, 1]), np.max(X_reduced[:, 1])))

z = (-theta[0] * xx - theta[1] * yy - d) * 1. / theta[2]

fig = plt.figure(1, figsize=(8, 6))
ax = Axes3D(fig, elev=-150, azim=110)

ax.scatter(X[:100, 0], X[:100, 1], X[:100, 2], c=y_train, cmap=plt.cm.Set1, edgecolor='k', s=40)
ax.plot_surface(xx, yy, z, alpha = 0.5)
plt.show()

enter image description here

I guess d in plane equation (ax+by+c*z = d) shouldn't be equal to 0. So I'm completely confused about this.

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  • $\begingroup$ See this answer for interactive 3D plots, 10814. The same can be done for the iris data. $\endgroup$ Commented May 22, 2020 at 22:09

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This answer is borrowed from-

https://stackoverflow.com/questions/36232334/plotting-3d-decision-boundary-from-linear-svm

from sklearn.svm import SVC
import numpy as np
import matplotlib.pyplot as plt
from sklearn import svm, datasets
from mpl_toolkits.mplot3d import Axes3D

iris = datasets.load_iris()
X = iris.data[:, :3]  # we only take the first three features.
Y = iris.target

#make it binary classification problem
X = X[np.logical_or(Y==0,Y==1)]
Y = Y[np.logical_or(Y==0,Y==1)]

model = svm.SVC(kernel='linear')
clf = model.fit(X, Y)

# The equation of the separating plane is given by all x so that np.dot(svc.coef_[0], x) + b = 0.
# Solve for w3 (z)
z = lambda x,y: (-clf.intercept_[0]-clf.coef_[0][0]*x -clf.coef_[0][1]*y) / clf.coef_[0][2]

tmp = np.linspace(-5,5,30)
x,y = np.meshgrid(tmp,tmp)

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')
ax.plot_surface(x, y, z(x,y))
ax.view_init(30, 60)
plt.show()
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