Chronogram min-overlays

For each interaction type in all of the evidence sources, if both vertices of the interaction had chronograms available, the chronograms were overlayed and the min of the value at each pixel was taken. This should indicate the minimum level of co-expression of the two interaction vertices at particular spatio-temporal points. (For phenotype and microarray datasets, only interactions where both vertices have high-confidence (HC) chronograms are used. The large number of interactions for these datasets gives too many chronogram overlays to present in a tractable manner.)