Tapio Wirkkala, glasservis "Tapio", 56 dlr, modell 2101. Iittala. 1970-tal.

Each sale is normalised to a neutral single item — hammer price divided by empirically measured quantity and condition factors (a pair ≈ 1.41× a single, damaged ≈ 0.82×) — the same recipe as the Tupendan valuation engine. Trend is a Theil–Sen fit over monthly medians; under ±5%/yr we call it stable.