def add_errorbar(mpl_axes, x_pos, y_pos, 
                 std, color = "grey", x_offset = 0.05, 
                 transparency = 0.75):
    """
    Adds error bars to MatPlotLib axes object. Modifies the mpl_axes input, returns None.

    mpl_axes -> matplotlib axes object
    x_pos    -> position of the center of the error bars on the x axis
    y_pos    -> position of the center of the error bars on the y axis
    std      -> standard deviation of the data (or vertical offset for error bars)
    color    -> The color of the error bars (Default: "grey")
    x_offset -> Size of the middle of the error bars (Default: 0.05)
    """
    # Plot the vertical middle bar
    mpl_axes.plot([x_pos, x_pos], [y_pos + std, y_pos - std], color = color, alpha = transparency)
    # Plot the horizontal middle bar
    mpl_axes.plot([x_pos - x_offset, x_pos + x_offset], [y_pos, y_pos], color = color, alpha = transparency)
    # Plot the horizontal top bar
    mpl_axes.plot([x_pos - 0.75*x_offset, x_pos + 0.75*x_offset], [y_pos + std, y_pos + std], color = color, alpha = transparency)
    # Plot the horizontal bottom bar
    mpl_axes.plot([x_pos - 0.75*x_offset, x_pos + 0.75*x_offset], [y_pos - std, y_pos - std], color = color, alpha = transparency)
    return None