Add support for captions and subtitles

This example shows the difference between ggplot and ggplotly objects regarding the use of "labs".
Source: R for Data Science

library(ggplot2)
library(plotly)
p<-ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(color = class)) +
  geom_smooth(se = FALSE) +
  labs(
    title = "Fuel efficiency generally decreases with engine size",
    subtitle = "Two seaters (sports cars) are an exception because of their light weight",
    caption = "Data from fueleconomy.gov"
  );
p  # This is ok, a ggplot2 object
g <- ggplotly(p)
g  #  "labs()" Just showing title (centered) and  subtilte & caption are not even showing up

labs() OK

labs() not ok

未指定實作檔案或測試。先重現所提供的 ggplot2 和 ggplotly 範例，然後追蹤 labs() 的轉換路徑；當結果圖如來源 ggplot 所示同時顯示副標題和圖說時，即視為完成。