The straight line modeled in your code is a line going diagonally through space and not through the center of the ellipse. The spiral rises along the z-axis as 't' increases. You can replace the definition of the line with:
x2 = zeros(size(t));
y2 = zeros(size(t));
z2 = t;
This will plot a straight line along the z-axis, up the center of the ellipse. Below is the updated code:
t = linspace(0, 8*pi, 100);
x = 6*cos(t);
y = 4*sin(t);
z = t;
x2 = zeros(size(t));
y2 = zeros(size(t));
z2 = t;
for k=1:length(t)
clf
t_k = t(k);
x_k = x(k);
y_k = y(k);
z_k = z(k);
x2_k = x2(k);
y2_k = y2(k);
z2_k = z2(k);
plot3(x_k, y_k, z_k, 'bo', 'LineWidth', 2, 'MarkerSize', 16)
hold on
plot3(x, y, z, 'b-', 'LineWidth', 2)
hold on
plot3(x2_k, y2_k, z2_k, 'ro', 'LineWidth', 2, 'MarkerSize', 16)
hold on
plot3(x2, y2, z2, 'r-', 'LineWidth', 2)
grid on
xlabel('x')
ylabel('y')
zlabel('z')
title('Planetary Orbit')
axis equal
drawnow
end
