Error using plot, data must be numeric, array convertible to double

Why is it giving error for the last line to plot?

Error using plot: data must be numeric datetime duration or an array convertible to double.

I want to plot the graph for cost and internal length

        x1 = input('Input volume of tank in gallons: ');
        x2 = x1/7.48;
        syms int_l;
        
cost = (2*int_l.^2+4.*(int_l+0.5)*(x2./(int_l.^2)+1))*812.75;
diff_cost = diff(cost);

answ = solve(diff_cost == 0);
int_l = answ(isAlways(answ>0));
int_l = double(int_l);
int_h = x2/(int_l.^2);
disp('----------------------------------------------------------------')
disp(['Length of the internal square base of the tank in feet will be: ' num2str(int_l)])
disp(['Length of the internal height of the tank in feet will be: ' num2str(int_h)])

ext_l = int_l + 1;
ext_h = x2./(int_l.^2)+1;
disp('----------------------------------------------------------------')
disp(['Length of the external square base of the tank in feet will be: ' num2str(ext_l)])
disp(['Length of the external height of the tank in feet will be: ' num2str(ext_h)])
        
plot(cost,int_l);
grid on;
        
        
        

Answer 1

[Walter Roberson]

syms int_l;

int_l is symbolic

cost = (2*int_l.^2+4.*(int_l+0.5)*(x2./(int_l.^2)+1))*812.75;

cost is in terms of symbolic int_l so cost will be symbolic.

You do not change cost after that, so it is still symbolic when you get to

plot(cost,int_l);

However, there is no plot function defined for symbolic expressions. There is a fplot function for symbolic expressions. When you use fplot() if you provide a second parameter then the second parameter must be the range of values to do the plotting over.