grid.rect(name="r1")
rg1 <- radialGradient(c("white", "black"))
grid.gradientFill("r1", rg1)


grid.rect(name="r2")
rg2 <- radialGradient(c("white", "black", "white"),
                      stops=c(0, .25, 1),
                      fx=.25, fy=.5)
grid.gradientFill("r2", rg2)


pdf(NULL, width=6, height=1.5)
# grid.newpage()
pushViewport(viewport(x=1/4, width=.4, height=.8))
grid.rect(name="r1")
rg1 <- radialGradient(c("white", "black"))
grid.gradientFill("r1", rg1)
popViewport()
pushViewport(viewport(x=3/4, width=.4, height=.8))
grid.rect(name="r2")
rg2 <- radialGradient(c("white", "black", "white"),
                      stops=c(0, .25, 1),
                      fx=.25, fy=.5)
grid.gradientFill("r2", rg2)
popViewport()
grid.export("Figures/export-radialgradient.svg")
dev.off()
system("convert Figures/export-radialgradient.svg Figures/export-radialgradient.pdf")
system("convert Figures/export-radialgradient.pdf Web/export-radialgradient.png")