pgfplots：如何在多轴图中移动图例

Question

此图似乎总是在右上角东北位置生成图例。我该如何将其移动到东南方向？将图例样式传递给图本身似乎不起作用。

\begin{tikzpicture}

  \pgfplotsset{
      scale only axis,
  }

  \begin{axis}[
    axis y line*=left,
    xlabel=Steps,
    ylabel= BLEU,
  ]
    \addplot[mark=*, color=blue]
      coordinates{
        (1, 0.00012801506090909243)(2372, 0.07797393202781677)(4726, 0.4955924153327942)
      }; \label{plot_1_y1}

    \end{axis}

    \begin{axis}[
      axis y line*=right,
      axis x line=none,
      ylabel= CHRF,
    ]

    \addplot[mark=x, color=red]
      coordinates{
        (1, 0.06872139871120453)(2372, 0.04961954057216644)(4726, 0.060143500566482544)(7070, 0.0851062685251236)

      }; \label{plot_1_y2}
    
    \addlegendimage{/pgfplots/refstyle=plot_1_y1}\addlegendentry{$y_1$}
    \addlegendimage{/pgfplots/refstyle=plot_1_y2}\addlegendentry{$y2$}
  \end{axis}

\end{tikzpicture}