fix plots; ggeffects::ggpredict() full.data arg is deprecated

[jdunic]

I get the following error when plotting the model predictions with simulated observed values

ggeffects::ggpredict(m_poisson, "x", full.data = TRUE) %>%
   ggplot(aes(x, predicted)) +
   geom_line(colour = "red") + 
   geom_ribbon(aes(ymin = conf.low, ymax = conf.high), alpha = .2) +
   geom_point(aes(y = observed))

Error in FUN(X[[i]], ...) : object 'observed' not found

• full.data is no longer an argument in ggeffects::ggpredict() and so 'observed' data no longer exists
• see https://github.com/strengejacke/ggeffects/issues/193

Here is a possible fix

[seananderson]

Thanks Jillian!