Why is there an error on my plot line?

I am writing a code that plots Lagrange-Polynomial-Interpolation and the part that's messing up is the plot(X,P,'b');

I have inserted a % to make it a comment and test the rest of the code and the rest of the code works. But when I remove the % from the plot(X,P,'b'); line, an error pops up at this line. What am I doing wrong?

Here's the entire code:

f = @(x) 3*exp(-x)+2*sin(x);
X = 0:3;
Y = f(X);
n = length(X);
P = 0;
syms x;
L = sym(zeros(size(X)));
for i = 1:n
    L(i) = 1;
    for j = 1:n
        if (j ~= i)
            L(i) = L(i)*(x-X(j))/(X(i)-X(j));
        end 
    end
    P = P + Y(i)*L(i);
end
disp(P)
plot(X,P,'b');
hold on;
grid on;
fplot(f,[0,3],'r');
hold on;
plot(X,Y,'or');

Answer 1

[KSSV]

f = @(x) 3*exp(-x)+2*sin(x);
X = 0:3;
Y = f(X);
n = length(X);
P = 0;
syms x;
L = sym(zeros(size(X)));
for i = 1:n
    L(i) = 1;
    for j = 1:n
        if (j ~= i)
            L(i) = L(i)*(x-X(j))/(X(i)-X(j));
        end 
    end
    P = P + Y(i)*L(i);
end
disp(P)

P = double(subs(P,X));
plot(X,P,'b');
hold on;
grid on;
fplot(f,[0,3],'r');
hold on;

plot(X,Y,'or');

[Paul Jones]

It worked! what is the reason to use double(subs as opposed to what I was doing?