DF = data.frame(
    city=c( 'A', 'B', 'C', 'D' ),
    abc=c( 19.7, 18.6, 19.1, 17.9 ),
    cbs=c( 16.1, 15.8, 14.6, 17.1 ),
    nbc=c( 18.2, 17.9, 15.3, 18.0 )
)

library(reshape2)
DF_melt = melt(DF, id.vars=c('city'), value.name='share', variable.name='network')