\usetikzlibrary{arrows,intersections}
\tikzpicture[
thick,
>=stealth',
dot/.style = {
draw,
fill=white,
circle,
inner sep=1pt,
minimum size=7pt
}
]
\coordinate (O) at (0,0);
\draw[->] (-0.3,0) -- (8,0) coordinate[label={below:$t$}] (xmax);
\draw[->] (0,-0.3) -- (0,7) coordinate[label={right:$y$}] (ymax);
\path[name path=x] (0,0) -- (7,7);
\path[name path=y] plot[smooth] coordinates {(-0.3,2) (2,1.5) (4,2.8) (6,5)};
\scope[name intersections={of=x and y,name=i}]
\draw (0,0) -- (5,5) node[pos=1,below right] {f(t)=t};