How to log transform values for color in Plotly but to keep original values on colorbar?

Question

I am trying to log transform values that are defining color on a Plotly graph, but I would like to keep original values on the Plotly color bar legend (not log-transformed numbers) in order to improve readability.

Here is the example of what I am trying to do on the mtcars data-set:

mtcars %>% plot_ly(x = ~hp, 
               y = ~qsec,                                                                        
               size = ~disp,
               color = ~mpg)

and you will get this graph:

Let's say I want to log transform color variable (mpg) with this code:

mtcars %>% plot_ly(x = ~hp, 
               y = ~qsec,                                                                        
               size = ~disp,
               color = ~log(mpg))

I will get this graph:

I am satisfied now with the graph, but now the colorbar on the right is having log() numbers.

My question is: how to log() transform color variable on a graph but keep the original numbers on the colorbar that are appropriately adjusted to new log colors?

So, on the one hand, I would like to have the original numbers on the second picture color bar, instead of 2.5, 3 and 3.5, but on the other hand, I would like to keep the color-positions of these numbers as they are on the log scale and without using ggplotly.

Answer 1

Similarly to this answer, this is a matter of using transformed values for the colour bar ticks, but untransformed values as labels.

Here is an option:

library(dplyr)
library(plotly)

# Define pretty breaks on transformed scale
brks_transformed <- pretty(log10(mtcars$mpg), n = 5)

# Breaks on the untransformed scale
brks_untransformed <- sprintf("%.1f", 10^brks_transformed)

mtcars %>% 
    plot_ly(
        x = ~hp, y = ~qsec, size = ~ disp, fill = ~ "",
        type = "scatter",
        mode = "markers",
        marker = list(
            color = ~ log10(mpg),
            line = list(width = 0),
            colorbar = list(
                tickmode = "array",
                ticktext = brks_untransformed,
                tickvals = brks_transformed)))