Allow dates when selecting "typical" values.

[dtkaplan]

Currently, dates are treated as categorical, and so we fail to interpolate when nlevels = Inf and the result can't be handed off to evaluate.

[rpruim]

Not sure I have fixed everything, but I've added code so that typical_values() can handle dates:

library(mosaicModel)
#> Loading required package: mosaicCore
#> Loading required package: splines
#> Loading required package: magrittr
library(mosaicData)
library(ggformula)
#> Loading required package: ggplot2
#> 
#> New to ggformula?  Try the tutorials: 
#>  learnr::run_tutorial("introduction", package = "ggformula")
#>  learnr::run_tutorial("refining", package = "ggformula")
theme_set(theme_bw())
bmod <- lm(births ~ date * wday, data = Births2015)
mod_eval(bmod, nlevels = 3)
#>         date  wday model_output
#> 1 2015-03-11 Thurs    11994.204
#> 2 2015-07-01 Thurs    12082.644
#> 3 2015-10-22 Thurs    12171.874
#> 4 2015-03-11   Sun     7254.654
#> 5 2015-07-01   Sun     7397.171
#> 6 2015-10-22   Sun     7540.962
#> 7 2015-03-11   Mon    11465.407
#> 8 2015-07-01   Mon    11735.764
#> 9 2015-10-22   Mon    12008.534
mod_plot(bmod)

gf_point(births ~ date, data = Births2015, color = ~wday) %>%
  mod_plot(bmod, nlevels = 7)

Created on 2018-06-22 by the reprex package (v0.2.0).