Specify boxmode to fix grouped boxplots

Filled boxplots did not have position = dodge in plotly - instead,
all boxes appear on top of each other. This can be fixed by setting
the layout of boxmode to group, but it also produces a number of
warnings. These warnings can apparently be ignored, but I haven't
found a good way to do that yet. (suppressWarnings seems dangerous,
and expect_warning makes the test code cluttered.)

Info about this warning:
plotly/plotly.R#1644
https://community.plotly.com/t/not-a-real-error-for-boxmode/2596

Author: amstilp

R/mod_plot.R

@@ -46,7 +46,8 @@ mod_plot_server <- function(id, dataset, selections){
     })
 
     output$plot <- renderPlotly({
-      ggplotly(plot_obj())
+      req(plot_obj)
+      plot_obj() %>% ggplotly() %>% plotly::layout(boxmode = 'group')
     })
   })
 }