The set of colors of the lines in the legend doesn't match with the set of colors of the circles because the xlines and ylines are also being used in the legend. To fix that, you can make the legend based on the circles only.
K = [100;200;400;600;800;1000;1200;1400;1600;1800;2000];
a = 1;
B=0.2;
r = radius_cal (K,B) ;
figure
hold on % (you can do "hold on" only once, at the top)
yline(0)
h = zeros(1,numel(r)); % h: array of circle lines
for i = 1:numel(r)
h(i) = circle (0,0,r(i),K(i));
xline (r(i))
end
yline (a,'Color','b')
fontSize = 14;
xlabel('radius')
legend(h) % legend based on the lines in h only
grid minor
function r = radius_cal (K,B)
E0= my_const.m_mu_kg .* my_const.C .* my_const.C .* 6241506479963.2 ;
gamma = 1+ (K ./ E0);
v = my_const.C * sqrt (1- (1./(gamma .* gamma)));
omega= ((my_const.q .* B) ./ my_const.m_mu_kg) .* sqrt (1- (v .* v) ./ (my_const.C .* my_const.C) );
r= v ./ omega;
end
function h = circle(x,y,r,K)
% pass in a K value to be used as the DisplayName
% of the line (for the legend)
ang=0:0.001:2*pi / 4;
xp=r*cos(ang);
yp=r*sin(ang);
% return the plotted line (h):
h = plot(x+xp,y+yp,'DisplayName',num2str(K));
end
