def linear_interpolate(x_axis, y_axis, x_new_axis, enable_warning=True):
    """Interpolate a y = f(x) function using linear interpolation.

    A smart way to interpolate arbitrary-range x_new_axis. The trick is 
    to add one more point to the original x_axis at x_new_axis[0] and 
    x_new_axis[-1], if x_new_axis is out of range.
    """
    left_pad_x, left_pad_y = list(), list()
    right_pad_x, right_pad_y = list(), list()

    if x_new_axis[0] < x_axis[0]:
        if enable_warning:
            print("WARNING! the first element of x_new_axis is at left "
                  "of x_axis. Use linear_interpolate(enable_warning=False) "
                  "to disable this warning.")
        left_pad_x.append(x_new_axis[0])
        left_pad_y.append(locate(x_axis[0], y_axis[0],
                                 x_axis[1], y_axis[1], x_new_axis[0]))

    if x_new_axis[-1] > x_axis[-1]:
        if enable_warning:
            print("WARNING! the last element of x_new_axis is at right "
                  "of x_axis. Use linear_interpolate(enable_warning=False) "
                  "to disable this warning.")
        right_pad_x.append(x_new_axis[-1])
        right_pad_y.append(locate(x_axis[-1], y_axis[-1],
                                  x_axis[-2], y_axis[-2], x_new_axis[-1]))

    if not ((len(left_pad_x) == 0) and (len(right_pad_x) == 0)):
        x_axis = left_pad_x + x_axis + right_pad_x
        y_axis = left_pad_y + y_axis + right_pad_y

    return rigid_linear_interpolate(x_axis, y_axis, x_new_axis)