Specify column grouping for color palettes

[jmlondon]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

During the posit::conf 2024 workshop we discussed how the color palette domain (when not specified) is calculated based on the minimum and maximum values within each column. This makes sense as a default given gt doesn't know whether particular columns share a domain so best to presume each column has an independent domain. There are situations, however, when the visualization would benefit from columns sharing a domain so values are consistent and comparable across columns. The initial suggestion in our discussion was to add some sort of grouping argument that could accept a list of columns (or tidy select function e.g. starts_with, ends_with,contains). In this situation, gt would calculate the domain based on the range of values across all of those columns and, then, create the color gradient.

This is currently possible with a custom function that calculates the domain and, then, that range is passed along to the domain argument. Integrating this within data_color() would be more user friendly.

example possible code

gt(exibble) |> 
data_color(columns = c(currency,num),
           palette = c("red","blue")#,
           #grouping = c(currency,num)
)