function [] = GantryDisplay(X, Xe, Ref)
x = X(1);
q = X(2);
% cart
xc = [-0.2,0.2,0.2,-0.2,-0.2] + x;
yc = [0,0,0.2,0.2,0];
xm = x + 2*sin(q);
ym = 0 - 2*cos(q);
% ball
q = [0:0.1:1]' * 2*pi;
xb = 0.05*cos(q) + xm;
yb = 0.05*sin(q) + ym;
clf;
plot([-3,3],[-2.5,2.5],'.');
hold on
plot([-3,3],[0,0],'b-');
plot(xc, yc,'r-');
plot([x,xm],[0,ym],'r-');
plot(xb, yb, 'r-');
plot([Ref, Ref],[-0.1,0.1],'b')

% Observer
x = Xe(1);
q = Xe(2);
% cart
xc = [-0.2,0.2,0.2,-0.2,-0.2] + x;
yc = [0,0,0.2,0.2,0];
xm = x + 2*sin(q);
ym = 0 - 2*cos(q);
% ball
q = [0:0.1:1]' * 2*pi;
xb = 0.05*cos(q) + xm;
yb = 0.05*sin(q) + ym;

plot(xc, yc,'g-');
plot([x,xm],[0,ym],'g-');
plot(xb, yb, 'g-');

pause(0.01);
end