def virtual_start(x_start, slope_start, y_base, y_start):
    """
    Calculate virtual start position. The trigger point will be projected to the base value by the slope at the trigger
    position.

    Parameters
    ----------
    x_start: float
        trigger position
    slope_start: float
        slope at trigger point
    y_base: float
        base value of the virtual start point
    y_start: flaot
        trigger value

    Returns
    -------
    virtual start poristion: float
    """
    return (y_base - y_start) / slope_start + x_start