When in doubt, vectorise every multiplication, exponention, and division.
With those changes (and nothing else) it works —
a = 1.7;
b = 1.3;
c = 0.9;
L = 2000; % Matrix size (1x2000)
l = 0 : (L-1);
x = b + l*((a-b)/L);
y = (c./(2*pi*(sqrt((a*a)-(x.^2)))).*(2*atan(sqrt((x.^2)-(b*b))./sqrt((a*a)-(x.^2)))));
figure
plot(x,y,'Color',[0,0.7,0.9])
title('2-D Line Plot')
xlabel('x')
ylabel('y')
I vectorised everything not already vectorised except a*a and b*b, since they obviously do not need vectorisation.
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