My fault, wrong wording. If you go backwards you see a 30% price increase per year which is a 23% cost reduction when you go forward (1/1.3 = ca. 0,77) which equates to a factor of about 10000 over 44 years.
Photovoltaics system costs in 1969 where about 3000$/kwp compared to about 1.4$/kwp in 2013. This is a factor of 2150 (19% improvement per year). Together with shorter module liveteam and higher maintainance cost and much higher installation area you get the 23% improvement.
I currently can't find 1969 data, but here is a plot going back to 1972:
I currently don't find long term data for coal, but there is a slow increase since 1995. The gap between coal and solar has been rapidly closing.
It is not guaranteed that the same cost can be reached, but we are allready down to a factor of 3, coming from a factor of several thousand. Currently no significant slow down in improvement is apparent. There is no data to support that coal will continue to be less expensive than solar twenty years from now. Denying this is similar to those who in the face of moores law claimed that there never could be interactice 3D computer graphics with bump mapping.