[R] ggplot 3-color gradient scales

[April Ettington]

Currently I am using these settings in ggplot to make a gradient from red
to blue.

geom_point( aes(x, y, color=z) ) +
scale_colour_gradient(low = "red",high = "blue") +

z is a ratio, and currently I am able to identify which have high and low
values, but I'd really like to be able to distinguish which are >1, <1, or
close to 1 by color.  It would be great if I could set a middle color in
this gradient (eg. green) that is set the the value of 1, even if that is
not the exact midpoint between my highest and lowest values.  Is there a
way to do this in R?

Thank you,
April

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