为图例添加颜色会导致条形图全部为正

Question

using Gadfly, DataFrames

v = DataFrame(x = [1,2,3], y = [0.5,-0.7,1.2], label = ["a","b","c"])

plot(v , x=:x, y=:y, Geom.bar, Theme(bar_spacing = 2mm), 
Guide.title("geneX"), Guide.ylabel("fold change"),
Guide.xlabel("technique"), color = :label)

产生以下图：

（snd bar应该是否定的）

#  without 'color = :label'
plot(v , x=:x, y=:y, Geom.bar, Theme(bar_spacing = 2mm),
Guide.title("geneX"), Guide.ylabel("fold change"), Guide.xlabel("technique"))

是否有针对此问题的解决方法？

致以最诚挚的问候，

塞巴斯蒂安