Plot Data to find the decision boundary for logisitc regression.

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Vivian Chow
Vivian Chow on 23 Feb 2021
Commented: Vivian Chow on 28 Feb 2021
Figure 1
I am using the Wisconsin Breast Cancer Diagnostic Data Set found on the UCI Machine Learning website. I am trying to implement logistic regression to classify the tumour as either malignant or benign, I changed the class to 0 for benign and 1 for malignant, for two features only, Marginal Adhesion and Clump Thickness. This is for an assignment at uni and we did not study much beyond logistic regression. I am aware there are other classification techniques out there but that is beyond the scope of my skills. I am trying to plot Maginal Adhesion against Clump Thickness however my graph ends up looking like the one above. I have used the following code that I found online.
This is the code to plot the data in a graph.
function plotData(X,y)
figure; hold on
pos = find(y==1); %Malignant
neg = find(y==0); %Benign
plot(X(pos,1), X(pos, 2), 'k+','LineWidth',5,'MarkerSize',5);
plot(X(neg,1), X(neg,2), 'ko','MarkerFaceColor','b','MarkerSize',5);
hold off;
end
Edit: I have included the code I used to plot the Decision Boundary, sorry I should have included that before too.
function plotDecisionBoundary(theta, X, y)
%Plot Data
plotData(X(:,2:3), y);
hold on
if size(X, 2) <= 3
%Only need 2 points to define a line, so choose two endpoints
plot_x = [min(X(:,2))-4, max(X(:,2))+4];
%Calculate the decision boundary line
plot_y = (-1./theta(3)).*(theta(2).*plot_x + theta(1));
%Plot and adjust axes for better viewing
plot(plot_x, plot_y)
%Legend
legend('Benign','Malignant', 'Decision Boundary')
axis([1,10, 1, 10])
else
%Here is the grid range
u = linspace(-1, 1.5, 50);
v = linspace(-1, 1.5, 50);
z = zeros(length(u), length(v));
%Evaluate z = theta*x over the grid
for i = 1:length(u)
for j = 1:length(v)
z(i,j) = mapfeature(u(i), v(j))*theta;
end
end
z = z'; %important to transpose z before contour
%Plot z = 0
%Notice you need to specify the range [0,0]
countour(u, v, z, [0, 0], 'LineWidth', 2)
end
hold off
end
To be honest, I am not too sure whether the code is right or not as I am still quite new to Matlab and am trying to get it working from watching videos online.
I want my graph to look like the following below; can anyone let me know what I am doing wrong?
Any help is appreciated. Thank you in advance.
Figure 2
  6 Comments
Vivian Chow
Vivian Chow on 28 Feb 2021
Thank you so much for your help! This defintely cleared up my confusion as to why my graph always looked different.

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Answers (1)

Gaurav Garg
Gaurav Garg on 26 Feb 2021
Hi Vivian,
You can look at the following link, where you can get more information on plotting model.

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