* Formulas (instead of plain expressions) are now required when using variable mappings. For example, `plot_ly(mtcars, x = wt, y = mpg, color = vs)` should now be `plot_ly(mtcars, x = ~wt, y = ~mpg, color = ~vs)`. This is a major breaking change, but it is necessary to ensure that evaluation is correct in all contexts (as a result, `evaluate` argument is now deprecated as it is no longer needed). It also has the benefit of being easier to program with (i.e., writing your own custom functions that wrap `plot_ly()`) since it preserves [referential transparency](https://en.wikipedia.org/wiki/Referential_transparency). For more details, see the [lazyeval vignette](https://github.com/hadley/lazyeval/blob/master/vignettes/lazyeval.Rmd)
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