...the source can be isolated to a series used by H.H. Lamb, representative of central England, last published (as figure 30 on p. 84) by Lamb (1982)... The ‘Central England’ curve also appeared in Lamb (1965: figure 3 and 1977: figure 13.4), on both occasions shown as an ‘annual’ curve together with the extreme seasons: winter (December to February) and high summer (July and August). The IPCC diagram comes from the 1982 publication as the vertical resolution of the annual plot is greater. The data behind the 1977 version are given in table app. V.3 in Lamb (1977), but these are essentially the same as previously given in Lamb (1965)... The 1982 version dispenses with the three possible curves evident in Lamb (1965, 1977) and instead uses a version which accounts for the ‘probable under-reporting of mild winters in Medieval times’ and increased summer temperatures to meet ‘certain botanical considerations’. Lamb (1965) discusses the latter point at length and raised summer temperatures in his Mediaeval reconstructions to take account of the documentary evidence of vineyards in southern and eastern England. The amount of extra warmth added during 1100–1350 was 0.3–0.4°C, or about 30% of the range in the black curve in Figure 7. At no place in any of the Lamb publications is there any discussion of an explicit calibration against instrumental data, just Lamb’s qualitative judgement and interpretation of what he refers to as the ‘evidence’. Variants of the curves also appear in other Lamb publications (see, eg, Lamb, 1969).So the point - in case you missed it - is that Lamb happily and with no real explanation other than personal taste adjusted his data. And - but you knew this already of course - not one of the "skeptics" either knows or cares.
* More use and abuse of IPCC 1990 fig 7.1(c).
* Adoration of the Lamb.