MATLAB: ERROR using plot. Vectors must be same lenght.

plotting error

I have a problem with the final part of this code, I can't plot the last graphic. Thank you. 
n=2000; % [rpm]
phi=0:2*pi/1000:2*pi; % [rad]
r=0.25; % [m]

l=0.57; % [m]
lam=r/l; % lambda
sb=r*(1+lam/4-cos(phi)-(lam*cos(2*phi))/4); % spostamento del pistone
syms x(ang);
x(ang)=r*(1+lam/4-cos(ang)-(lam*cos(2*ang))/4);
sb_sym=x(phi);
figure(1)
plot(phi,sb,'*',phi,sb_sym)
title('Grafico Spostamenti');
legend('spostamento [m]','spostamento:formula simbolica [m]');
grid on;
figure(2)
fplot(x,[0,2*pi])
title('Grafico Simbolico Spostamenti [m]');
syms x_dot(ang)
x_dot(ang)=diff(x(ang))
Om=n*((2*pi)/60); % omega [rad/s]
vb_sym=Om*x_dot(phi);
vb_num=Om*(diff(sb)./diff(phi));
vb=Om*r*(sin(phi)+(lam*sin(2*phi))/2);
figure(3)
plot(phi,vb,'*',phi(1:end-1),vb_num,phi,vb_sym)
title ('Grafico Velocità')
legend('velocita [m/s]','velocita:formula simbolica [m/s]');
grid on;
ab=(Om^2)*r*(cos(phi)+lam*cos(2*phi));
ab_num=(Om^2)*(diff(vb)./diff(phi));
syms vb(ang);
vb(ang)=Om*r*(sin(ang)+(lam*sin(2*ang))/2);
vb_dot(ang)=diff(vb(ang));
ab_sym=vb_dot(ang);
figure(4)
plot(phi,ab,'*',phi(1:end-1),ab(1:end-1),ab_num,phi,ab_sym)           *ERROR using plot. Vectors must be same lenght
title('Grafico Accelerazione')
legend('accelerazione[m/s^2]','accelerazione:formula simbolica [m/s^2]');
grid on;

Best Answer

you appear to have two errors on that plot line:
1) ab_sym is a symbolic expression - did you mean to exclude this?
2) what did you intend for the x axis of the ab_num plot?
You can make it clearer what you are trying to do if you change that one line to:
plot(phi,ab,'*')
hold on
plot(phi(1:end-1),ab(1:end-1))
plot(phi(1:end-1), ab_num) %    assume x axis is phi(1:end-1)
% plot(phi,ab_sym) % not sure what intended
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