Problem with Legend command.

syms x 

f=log(x); 

hold on
T1=taylor(f,'Expansionpoint',2,'order',1)
T2=taylor(f,'Expansionpoint',2,'order',2)
T3=taylor(f,'Expansionpoint',2,'order',3)
T4=taylor(f,'Expansionpoint',2,'order',4)
x=-5:.1:5;
xx=subs(x);

yy1=subs(T1);
plot(xx,yy1,'m');
yy2=subs(T2);
plot(xx,yy2,'r');
yy3=subs(T3);
plot(xx,yy3,'c');
yy4=subs(T4);
plot(xx,yy4,'b');
legend('po(x)','p1(x)','p2(x)','p3(x)')
xlabel('x');
ylabel('y');
grid on

The problem is very silly and i think i am missing something elementary.Why the legend command not giving different colors to the p0,p1,p2,p3 in the image.Any help will be appreciated.

Answer 1

[Star Strider]

The problem is the first plot call.

Change it to:

fplot(yy1,[-5 5],'m');

and everything else works.

EDIT —

Also, the ‘xx’ assignment is not necessary. Simply delete it and use ‘x’ instead in the rest, or use fplot for the rest, as with the first one.

[Asir Tushar]

syms x 

f=log(x); 

hold on
T1=taylor(f,'Expansionpoint',2,'order',1)
T2=taylor(f,'Expansionpoint',2,'order',2)
T3=taylor(f,'Expansionpoint',2,'order',3)
T4=taylor(f,'Expansionpoint',2,'order',4)
x=-5:.1:5;

yy1=subs(T1);
fplot(yy1,[-5 5],'m');
yy2=subs(T2);
fplot(yy2,[-5 5],'r');
yy3=subs(T3);
fplot(yy3,[-5 5],'c');
yy4=subs(T4);
fplot(yy4,[-5 5],'b');
legend('po(x)','p1(x)','p2(x)','p3(x)')
xlabel('x');
ylabel('y');
grid on

i changed it just like you said. but this error pops out. what i am doing wrong this time ?

Error using fcnchk (line 107)
If FUN is a MATLAB object, it must have an feval method.

Error in fplot (line 61)
   fun = fcnchk(fun);

Error in taylor (line 13)
fplot(yy1,[-5 5],'m');

[Walter Roberson]

The first couple of versions that had fplot did not support symbolic variables. You might be using an older release. If so then instead of passing yy to fplot, call matlabFunction on yy and pass the result to fplot