The polarplot function won't give you the x and y data. So, you need to calculate it yourself.
I've made up r and theta values since you didn't provide yours.
theta = linspace(0, 2*pi, 100);
r = 1 + (1 + cos(theta))/2;
polarplot(theta, r)
% calculate x and y from r and theta
x = r.*sin(theta);
y = r.*cos(theta);
% plot x divided by y against theta
plot(theta, x./y) % note the ./ to get element-by-element divide
grid on
xlabel 'theta'
ylabel 'x/y'
Note that x divided by y actually goes to infinity when y = 0. Fortunately, the linearly spaced theta vector that I used didn't result in any y values that were exactly zero or we would have gotten an error. due to dividing by zero.
