Modify rainbow_plot to allow bare variables

[Rob J Hyndman]

Similar to fabletools::autoplot.tbl_ts
Test case:

aus_mortality |>
  dplyr::filter(State=="Victoria", Sex != "total") |>
  autoplot(Exposure/1000) +
  ylab("Population at 30 June (thousands)")

[Rob J Hyndman]

Completed in e3b439c