def calc_variance_pct(actual, plan):
    """
    Calculates the percentage variance between actual and plan values
    for reporting purposes.

    The percentage variance is only returned if it is economically
    meaningful and interpretable:
    - The plan value must not be zero
    - Actual and plan must have the same sign (no sign change)

    In cases where a percentage variance would be misleading
    (e.g. sign change from loss to profit), the function returns None
    and absolute variance should be used instead.

    Parameters
    ----------
    actual : float | int
        Actual (realized) value.
    plan : float | int
        Planned or budgeted value.

    Returns
    -------
    float | None
        Percentage variance relative to the absolute plan value,
        or None if the percentage variance is not meaningful.
    """

    variance = actual - plan
    if plan == 0:
        return None
    if actual * plan < 0:
        return None
    return variance / abs(plan)




def display_value(value, unit):
    """
    Formats a numeric KPI value for reporting output based on the configured
    display unit (e.g. €, T€, Mio. €).

    - Scales the input value according to DISPLAY_UNIT
    - Applies European number formatting (thousands separator, decimal comma)
    - Returns 'n/a' if the input value is None

    Parameters
    ----------
    value : float | int | None
        Raw KPI value in base currency (e.g. EUR).

    Returns
    -------
    str
        Formatted value including display unit, ready for dashboard display.
    """    
    
    if value is None:
        return "n/a"
    
    unit_factors = {
        "Mio. €": 1_000_000,
        "T€": 1_000,
        "€": 1,
    }

    factor = unit_factors.get(unit, 1)
    scaled = value / factor
    formatted = f"{scaled:,.2f}"
    formatted = (
        formatted
        .replace(",", "X")
        .replace(".", ",")
        .replace("X", ".")
    )
    
    return f"{formatted} {unit}"