Trouble With Graphing Functions

Hello...I am making a basic program that has the original function, the derivative, and the integral. I then am trying to have it graph them all on the same graph but for some reason it's not working. Every time I run the program an error comes up that says:

Error using plot Non-numeric data is not supported in 'Line'

Is this because I have x declared as a variable using syms x? If so, how do I keep x as a variable but still have it graph?

Thank you!

    clc
    clear

    syms x;     %Makes x a variable

    %Original Function
    O = 2*x;                 %THIS IS THE ONE THAT CAN BE CHANGED
    display('Original Function')
    pretty(simplify(O))      %Makes it nice

    %Find the Derivative of the Original Function
    D = diff(O);
    display('Derivative')
    pretty(simplify(D))              %Makes it nice

    %Find the Integral of the Original Function
    I = int(O);
    display('Integral')
    pretty(simplify(I))              %Makes it nice

    %Now, to graph them
    B = -10:1:10;   %Bounds of the graph

    plot(B,O,'k--',B,D,'g--',B,I,'r--');

    xlabel('x')
    ylabel('y')
    title('Original, Derivative, Integral')
    grid on       %Graph Grid on

[Star Strider]

A few observations:

• ‘O’ could be improved upon as a function name, since you could confuse it with zero (0),
• ‘D = diff(O);’ will leave you one element short, so use the gradient function instead,
• ‘I = int(O);’ will probably try to invoke the Symbolic Math Toolbox integration and will of course fail, so use trapz or cumtrapz instead.