\begin{tikzpicture}
\begin{axis}[
    xmin=0,xmax=10,
    ymin=0,ymax=10,
    legend style={legend pos=north west},
    axis lines = left,
    xlabel = $t$ (s),
    ylabel = {$v$} (m/s),
    ticks = none
]
\addlegendentry{$x$}
\addplot[samples=100,domain=0:10,pattern=north west lines,area legend, pattern color=lightgray] {x} \closedcycle;
\addplot[samples=100,domain=0:10]{x};
\addlegendentry{$a$}
\end{axis}
\end{tikzpicture}