gradientBBox <- linearGradient(c("black", "white", "black"), 
                               gradientUnits="bbox",
                               x0=0, y0=.5, x1=1, y1=.5)


grid.rect(1:2/3, 1:2/3, width=1/3, height=.2, name="r2")
grid.gradientFill("r2", gradientBBox)
grid.export()


pdf(NULL, width=2, height=1)
grid.rect(gp=gpar(col="grey"))
grid.rect(1:2/3, 1:2/3, width=1/3, height=.2, name="r2")
grid.gradientFill("r2", gradientBBox)
grid.export()
grid.export("Figures/export-gradient-fill-bbox.svg")
dev.off()
system("inkscape --export-pdf=Figures/export-gradient-fill-bbox.pdf Figures/export-gradient-fill-bbox.svg")
system("convert Figures/export-gradient-fill-bbox.pdf Web/export-gradient-fill-bbox.png")