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rggplot2scale

how can I force the scale in ggplot to be in the middle of the data range?

In the plot below, the range in a goes from 0.001 to 0.7 but the legend doesn't have the middle value in the cyan color. Is there a way I can force that the middle value in the range of a be in the middle of the colour scale? enter image description here

 a <- c(0.1, 0.05, 0.09, 0.001, 0.7, 0.002, 0.1, 0.015, 0.05, 0.013, 0.05, 0.0021,     0.05, 0.002, 0.5, 0.01, 0.2, 0.001)
 b <- c(1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9)
 depth <- seq(1:18)

 df <- data.frame(a, b, depth)

 colors <- c("darkblue", "blue", "cyan", "yellow", "red")

 range(df$a)
 
 > range(df$a)
 [1] 0.001 0.700

 ggplot(df, aes(x = b, y = depth, fill = a)) +
 geom_tile(width = 1, height = 1) +
 scale_y_reverse(name = "Depth (m)") +
 scale_fill_gradientn(colors = colors, limits = c(0.01, 0.3), oob = scales::squish) +
 labs(x = "Category", y = "Depth (m)", fill = "Value") +
 theme_minimal()
Solution

  • When setting the scale limits manually ...

    ## ggplot code + 
      scale_fill_gradientn( ..., limits = c(.01, 0.3)))

    ... make sure they fit the range of the color-mapped data (here: c(.001, .7), or leave the manual specification altogether, in which case ggplot takes care of it.

    works:

    ## ggplot code +
  scale_fill_gradientn(colors = colors,
                       limits = c(0.001, 0.07) # match value range or skip
                       oob = scales::squish
                       )