Solution to combine facetted legends breaks for continuous variable

I am trying to use plotly to create facetted plots - I do this by using group_map() to create my plots robustly.

The solution I currently have works in reducing duplicated legend labels when using discrete variables for color/size/etc but fail to work for continuous variables. I assume this is do to each facetted plot potentially having different total ranges for the continuous variable.

library(tidyverse)
library(plotly)

city_list = unique(txhousing$city) %>%  
  sample(8)

##solution works for discrete labels 
txhousing %>%  
  filter(month < 6, 
         city %in% city_list) %>% 
  group_by(month) %>% 
  group_map(~{
    plot_ly(.x, x=~year, y = ~sales, color = ~city,legendgroup = ~city,type = "bar"
            ,showlegend = (.y == "1")
            ) %>%
      layout(yaxis = list(
        title = paste0(c(rep("&nbsp;", 20),
                         paste("<b>", as.character(.y), "</b>"),
                         rep("&nbsp;", 20),
                         rep("\n&nbsp;", 3)),
                       collapse = ""))) %>%
      layout(barmode = 'stack')
  }
  ) %>%
  subplot(nrows = NROW(.), margin = .05, shareX = T, shareY = T, titleY = T) 

##solution fails to merge legends for continuous variable
txhousing %>%  
  filter(month < 6, 
         city %in% city_list) %>% 
  group_by(month) %>% 
  group_map(~{
    plot_ly(.x, x=~year, y = ~sales, color = ~volume,legendgroup = ~volume,type = "bar"
            ,showlegend = (.y == "1")
            ) %>%
      layout(yaxis = list(
        title = paste0(c(rep("&nbsp;", 20),
                         paste("<b>", as.character(.y), "</b>"),
                         rep("&nbsp;", 20),
                         rep("\n&nbsp;", 3)),
                       collapse = ""))) %>%
      layout(barmode = 'stack')
  }
  ) %>%
  subplot(nrows = NROW(.), margin = .05, shareX = T, shareY = T, titleY = T) 

まず、plot_ly()、group_map()、subplot()を使用して、提供されている2つのR再現コードを実行し、離散色の場合と連続色の場合を比較します。subplotがトレースと凡例をどのように結合するかを追跡します。連続変数の凡例が、ファセットされたプロットの動作を損なわずに結合されれば完了です。