MATLAB: Is the circle invisible

circle

I can not figure out why my circle is invisiable when I plot it (It is a regression). Am I missing something? It used to work earlier. 
x = [5; 4; -1; 1];
y = [3; 5; 2; 1];
% circle's equation:  x^2+y^2 = 2xc1+2yc2+c3
a = [2.*x,2.*y,ones(length(x),1)];
b = [x.^2 + y.^2];
c = a\b;
x_m = c(1)/2;
y_m = c(2)/2;
r = sqrt(x_m^2 + y_m^2 -c(3));
% plot data points
plot(x,y,'o')
hold on
% plot center
plot(x_m,y_m,'+')
% plot circle
circle(x_m,y_m,r)
hold off

Best Answer

viscircles([x_m, y_m], r)
axis equal
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