line: fix for 1st datapoint not being concrete
line: fix for 1st datapoint not being concrete

in such case the points_colors array would still be empty, although i>1

JuliaPackageMirrors/Gadfly.jl,zhmz90/Gadfly.jl,maximsch2/Gadfly.jl,dcjones/Gadfly.jl,kshramt/Gadfly.jl,Roger-luo/Gadfly.jl,peter1000/Gadfly.jl,Godisemo/Gadfly.jl,spencerlyon2/Gadfly.jl,joehuchette/Gadfly.jl,tbreloff/Gadfly.jl