function plotatom(x0,y0,color)

% plot one atom in the specified color, centered at (x0,y0) with radius r

r = 0.5;
theta = 0:0.1:6.29;
x = x0 + r*cos(theta);
y = y0 + r*sin(theta);

fill(x,y,color)
plot(x,y,color)