如何在R plotly色标中强制将零映射为白色

Question

My code is the following but I would like to have white color mapped to 0 instead of the middle values of the max and min of data_bydate$avg_score

g <- plot_ly(data_bydate, x = ~Date, y = ~avg_score, color =data_bydate$avg_score, colors = c("#FF0000","#FFFFFF", "#008000"), mode ='markers', marker = list(size=3)) %>%
layout(legend = list(orientation = 'h'), 
       xaxis = list(title = "",
                    zeroline = FALSE,
                    tickformat="%X",
                    showline = TRUE), 
       yaxis = list(title = "",
                    zeroline = FALSE,
                    showline = TRUE ))

Answer 1

You could create your own colorscale which goes from red (min) to white (zero) to greenish (max). The colorscale is a list of arrays which is composed of two separate colorscales, one for the part below zero, from red to white, and the other one above, from white to green.

library(plotly)
library(scales)

set.seed(42)
colorlength <- 100

data_bydate <- data.frame(Date = c(1:23),
                          avg_score = c(runif(10, -5, -1), 
                                        rep(0,3), 
                                        runif(10, 1, 10)
                                        )
                          )

null_value <- (0 - min(data_bydate$avg_score)) / (max(data_bydate$avg_score) - min(data_bydate$avg_score))        
border <- as.integer(null_value * colorlength)
colorscale <- as.list(1:colorlength)

#colorscale below zero
s <- scales::seq_gradient_pal("#FF0000", "#FFFFFF", "Lab")(seq(0,1,length.out=border))
for (i in 1:border) {
    colorscale[[i]] <- c((i - 1) / colorlength, s[i])
}

#colorscale above zero
s <- scales::seq_gradient_pal("#FFFFFF", "#008000", "Lab")(seq(0,1,length.out=colorlength - border))
for (i in 1:(colorlength - border)) {
  colorscale[[i + border]] <- c((i + border) / colorlength, s[i])
}

plot_ly(data_bydate, 
        x = ~Date, 
        y = ~avg_score, 
        type = 'scatter',
        mode ='markers', 
        marker = list(size = 10,
                      color = ~avg_score,
                      colorscale = colorscale,
                      colorbar = list(len=1))) %>%
  layout(plot_bgcolor = 'black')

Answer 2

A simple way is to set limits, here min is -2, max is 2, and 0 is in the middle.

colorbar(title = "Change (%)", limits = c(-2, 2))

And choose a diverging palette like;

colorscale = "RdBu",
reversescale = TRUE,

The diverging palettes are : BrBG, PiYG, PRGn, PuOr, RdBu, RdGy, RdYlBu, RdYlGn, Spectral.

Reversing the scale reverses the direction of the colorscale.

Answer 3

Just to add to Maximilian Peters' excellent answer, in some applications it may be desirable to have an asymmetric colour scale that reflects the asymmetry in the data. In the example in Maximilian's answer, this would mean that -4 would be of a shade of red comparable in brightness to the shade of green of value 4.

library(plotly)
library(scales)

set.seed(42)
colorlength <- 100

data_bydate <- data.frame(Date = c(1:23),
                          avg_score = c(runif(10, -5, -1), 
                                        rep(0,3), 
                                        runif(10, 1, 10)
                                        )
                          )

null_value <- (0 - min(data_bydate$avg_score)) / (max(data_bydate$avg_score) - min(data_bydate$avg_score))        
border <- as.integer(null_value * colorlength)
colorscale <- as.list(1:colorlength)


if (border < colorlength / 2) {
    ## colorscale below zero
    border_neg <- colorlength - border
    s <- scales::seq_gradient_pal("#FF0000", "#FFFFFF", "Lab")(seq(0,1,length.out=border_neg))
    for (i in 1:border) {
        colorscale[[i]] <- c((i - 1) / colorlength, s[i + border_neg - border - 1])
    }

    ## colorscale above zero
    border_pos <- border
    s <- scales::seq_gradient_pal("#FFFFFF", "#008000", "Lab")(seq(0,1,length.out=colorlength - border_pos))
    for (i in 1:(colorlength - border_pos)) {
        colorscale[[i + border_pos]] <- c((i + border) / colorlength, s[i])
    }
} else {
    ## colorscale below zero
    border_neg <- border
    s <- scales::seq_gradient_pal("#FF0000", "#FFFFFF", "Lab")(seq(0,1,length.out=border_neg))
    for (i in 1:border_neg) {
        colorscale[[i]] <- c((i - 1) / colorlength, s[i])
    }

    ## colorscale above zero
    border_pos <- colorlength - border
    s <- scales::seq_gradient_pal("#FFFFFF", "#008000", "Lab")(seq(0,1,length.out=border))
    for (i in 1:border_pos) {
        colorscale[[i + border]] <- c((i + border) / colorlength, s[i])
    }
}


plot_ly(data_bydate, 
        x = ~Date, 
        y = ~avg_score, 
        type = 'scatter',
        mode ='markers', 
        marker = list(size = 10,
                      color = ~avg_score,
                      colorscale = colorscale,
                      colorbar = list(len=1))) %>%
  layout(plot_bgcolor = 'black')