How to specify "low" and "high" and get two scales on two ends using scale_fill_gradient

You can try adding a white midpoint to scale_fill_gradient2:

gg <- ggplot(nba.s, aes(variable, Name))gg <- gg + geom_tile(aes(fill = rescale), colour = "white")gg <- gg + scale_fill_gradient2(low = "darkgreen", mid = "white", high = "darkred")gg <- gg + labs(x="", y="")gg <- gg + theme_bw()gg <- gg + theme(panel.grid=element_blank(), panel.border=element_blank())gg

But, you'll have the most flexibility if you follow the answer in the SO post you linked to and use scale_fill_gradientn.

EDIT (to show an example from the comment discussion)

# change the "by" for more granular levelsgreen_seq <- seq(-5,-2.000001, by=0.1)red_seq <- seq(2.00001, 5, by=0.1)nba.s$cuts <- factor(as.numeric(cut(nba.s$rescale,                              c(green_seq, -2, 2, red_seq), include.lowest=TRUE)))# find "white"white_level <- as.numeric(as.character(unique(nba.s[nba.s$rescale >= -2 & nba.s$rescale <= 2,]$cuts)))all_levels <- sort(as.numeric(as.character(unique(nba.s$cuts))))num_green <- sum(all_levels < white_level)num_red <- sum(all_levels > white_level)greens <- colorRampPalette(c("#006837", "#a6d96a"))reds <- colorRampPalette(c("#fdae61", "#a50026"))gg <- ggplot(nba.s, aes(variable, Name))gg <- gg + geom_tile(aes(fill = cuts), colour = "white")gg <- gg + scale_fill_manual(values=c(greens(num_green),                                      "white",                                       reds(num_red)))gg <- gg + labs(x="", y="")gg <- gg + theme_bw()gg <- gg + theme(panel.grid=element_blank(), panel.border=element_blank())gg <- gg + theme(legend.position="bottom")gg

The legend is far from ideal, but you can potentially exclude it or work around it through other means.