MATLAB: Vectors must be the same length with plot of Shear Force Diagram

plot error

I'm not famililar with the error. If I run the plot for the moment diagram it works, but not for the shear force. 
%% conversions
w = 4;       % VDL in k/ft
d = 12;      % Distance for VDL
P1 = w*d;    % VDL to Point Load in kip
a = 6;       % centroid of the point load
P2 = 60;
%% Reaction Forces
Ay = P1 + P2;
MA = P1*a + P2*20;
%% Consider the left of a section through a-a
x1 = linspace(0,12);
P3 = P1*x1;
% Equilibrium

Va = Ay - P3;
Ma = (Ay*x1)-(P3.*(x1/2))-MA;
%% Consider the right of a section through b-b
x2=linspace(12,20);
% Equilibrium
Vb = P2;
Mb = -60*(20-x2);
X = [x1 x2]; V = [Va Vb]; M = [Ma Mb];
subplot(211)
plot(X,V)
subplot(212)
plot(X,M)
-----------------------------
Error using plot
Vectors must be the same length.
Error in hibbler_SFBM_142 (line 32)
plot(X,V)

Best Answer

If you look in the workspace, you should see that X has length 200 while V has length 101. If you want to plot V over X, they need to have the same length.
You create X by combining x1 and x2, both of which have length 100. Thus X has length 200.
You create V by combining Va and Vb, and while Va has length 100, Vb has length 1. Thus V has length 101.
Do you mean for Vb to equal P2 over the entire duration of x2? Then you could define it with this:
Vb = P2*ones(size(x2));
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