Segments of Known Genetic DistanceMarkers used to generate the map reported in Liu et al. 2009 were used to estimate cM per bp. The full map is also available at MaizeGDB. For this calculation, markers were ignored in regions where RefGen_v2 and the genetic map were not collinear. The full dataset used for this calculation is available here. Markers removed from the original Liu et al. dataset are available as a list here.
How bars are drawn is best explained by example.
Regions where bp increment but no crossovers are observed are regions where the genetic distance is 0 cM and no bar is drawn.