Beamer: Graphic outside the frame

No matter what I have done (change the axis limits, scale, resize the witdh of the colums) the graph is always outside

begin{frame}frametitle{test}
begin{block}{}
D $x^2+4y^2-6x+16y+21=0$,
end{block}
begin{columns}
begin{column}{6cm}
begin{itemize}
item$x^2-6x+4y^2+16y=-21$
item$x^2-6x+9-9+4y^2+16y=-21$
item$(x-3)^2-9+4(y^2+4y+4-4)=-21$
item$(x-3)^2-9+4(y+2)^2-16=-21$
item$(x-3)^2+4(y+2)^2=-21+9+16=4$
item$frac{(x-3)^{2}+4(y+2)^{2}}{4}=frac{4}{4}=1$
item$frac{(x-3)^{2}}{4}+frac{(y+2)^{2}}{1}=1$



end{itemize}
end{column}

begin{column}{6cm}
linebreak
linebreak
pgfplotsset{
    every axis/.append style={
            axis x line=middle,
            axis y line=middle,
            xlabel={$x$},
            ylabel={$y$},
            axis line style={->},
        },
    marya/.style={color=green,thick,mark=none},
    soldot/.style={color=green,only marks,mark=*},
    holdot/.style={color=green,fill=white,only marks,mark=*},
    grid style={dotted,gray},
}

tikzset{>=stealth}
begin{flushleft}
begin{tikzpicture}
    begin{axis}[
            grid=both,
            axis equal,
            xmin=0,xmax=5,
            ymin=-4,ymax=0,
            {scale=0.75}
        ]
        draw[marya,domain=-3:6,samples=50] (3,-2) ellipse (2 and 1);
        addplot[soldot]coordinates {(-15,0)} node [anchor=north west,text=black]  {$f$};
        addplot[soldot]coordinates {(15,0)} node [anchor=north west,text=black]  {$f$};
    end{axis}
end{tikzpicture}
end{flushleft}
ss
end{column}
end{columns}
end{frame}

What can be done in order to appear completely in the column space?

documentclass{beamer}
usepackage{pgfplots}

begin{document}

begin{frame}
frametitle{Frametitle}
    begin{columns}
        begin{column}{0.5textwidth}
            begin{itemize}
                item Item
                item Item
            end{itemize}
        end{column}
        begin{column}{0.5textwidth}
            begin{tikzpicture}
                begin{axis}[
                    width = 1.0textwidth,
                    xlabel = $x$,
                    ylabel = {$f(x) = x^2 - x +4$},
                    ]
                addplot {x^2 - x +4};
                end{axis}
            end{tikzpicture}
        end{column}
    end{columns}
end{frame}

end{document}