要按图例选择子集数据并使用ggplotly的geom_bar函数进行可视化，可以按照以下步骤进行：

1. 导入必要的库：

library(ggplot2)
library(plotly)

2. 创建示例数据集：

df <- data.frame(
  x = c("A", "B", "C", "A", "B", "C"),
  y = c(10, 15, 12, 8, 6, 9),
  group = c("Group 1", "Group 1", "Group 1", "Group 2", "Group 2", "Group 2")
)

3. 使用ggplot函数创建柱形图：

p <- ggplot(data = df, aes(x = x, y = y, fill = group)) + geom_bar(stat = "identity")

4. 使用ggplotly函数将ggplot对象转换为交互式图形：

p <- ggplotly(p)

5. 在图例上添加交互式功能：

p <- p %>% event_register("plotly_legendclick")

6. 创建一个反应式函数来动态选择子集数据：

subset_data <- reactive({
  legend_click <- event_data("plotly_legendclick")
  if(is.null(legend_click)){
    return(df)
  } else {
    selected_group <- legend_click[["curveNumber"]] + 1
    subset_df <- df[df$group == unique(df$group)[selected_group], ]
    return(subset_df)
  }
})

7. 在图形上添加反应式层：

p <- p %>% event_reactive("plotly_legendclick", function() {
  subset_df <- subset_data()
  new_trace <- subset_df %>% plot_ly(x = ~x, y = ~y, type = "bar", color = ~group)
  p <- add_trace(p, new_trace)
})

完整的代码示例如下：

library(ggplot2)
library(plotly)

df <- data.frame(
  x = c("A", "B", "C", "A", "B", "C"),
  y = c(10, 15, 12, 8, 6, 9),
  group = c("Group 1", "Group 1", "Group 1", "Group 2", "Group 2", "Group 2")
)

p <- ggplot(data = df, aes(x = x, y = y, fill = group)) + geom_bar(stat = "identity")
p <- ggplotly(p)
p <- p %>% event_register("plotly_legendclick")

subset_data <- reactive({
  legend_click <- event_data("plotly_legendclick")
  if(is.null(legend_click)){
    return(df)
  } else {
    selected_group <- legend_click[["curveNumber"]] + 1
    subset_df <- df[df$group == unique(df$group)[selected_group], ]
    return(subset_df)
  }
})

p <- p %>% event_reactive("plotly_legendclick", function() {
  subset_df <- subset_data()
  new_trace <- subset_df %>% plot_ly(x = ~x, y = ~y, type = "bar", color = ~group)
  p <- add_trace(p, new_trace)
})

p

这将创建一个交互式柱形图，可以通过单击图例中的组来选择子集数据。