def change_linewidth(ax, lw=3):
    """change linewidth for each line plot in given axis
    
    Parameters
    ----------
    ax : mpl.axis
        axis to change fontsize of
    lw : float, optional
        [description], by default 3
    
    Returns
    -------
    ax
        mpl.axis 

    Examples
    --------

        fig, ax = plt.subplots(1, 1)
        x = np.arange(10)
        y = np.arange(10)
        ax.plot(x, y, x + 1, y, x -1, y )
        change_linewidth(ax, 3)
    """
    for item in ax.lines:
        item.set_linewidth(lw)
    return ax