I am plotting a non-linear function -2/3 x^ 3 + x^2-3x+4/3 in the interval [0,1/2], I tried to change the scales to make it look less linear but still looks like a straight line. Any suggestions?
\usepackage{tikz}
\begin{figure}
\begin{tikzpicture}[xscale=5,yscale=3,domain=0:1,step=.01];
\draw[<->] (0,1.5)--(0,-.3);
\draw[<->] (-.3,0)--(1,0);
\draw[] (1,0) node[right]{\small $x$};
\draw[color=blue] plot[samples=200] (\x,{ifthenelse(\x <.5,-(2/3)*\x^3+\x^2-3*\x+4/3,(2/3)*\x^3-\x^2+3*\x-4/3))}) node[right] {\small{$b(x)$}};
\draw[color=red] (-.2,0.635) node[right]{$\beta$};
\draw[color=red] (0,0.635)--(1,0.635);
\draw[color=red,dotted,thick](.25,-.3)--(.25,0.635);
\draw[color=red,dotted,thick](.75,-.3)--(.75,0.635);
\draw[color=red](.25,-.3)node[below]{\footnotesize{$\phi_1(\beta)$}};
\draw[color=red](.75,-.3)node[below]{\footnotesize{$\phi_2(\beta)$}};
\end{tikzpicture}
\end{figure}