ggvis plot with too many labels: how to only display every nth label?

Question

I have a ggvis plot in a Shiny app with too many x axis labels (see https://i.stack.imgur.com/lLppD.png).

In my ggplot implementation I called a function mylabels that would skip most labels and only display every nth label:

scale_x_discrete(labels=mylabels)

where

mylabels <- function(x) {
      y = c()
      marks = seq(1, length(x), 7)
      for (j in 1:length(x)) {
        if (j %in% marks) {
          y[j] = x[j]
        }
        else {
          y[j] = ""
        }
      }
      return (y)
    }  

This worked well in ggplot. I tried to implement the same solution in ggvis but to no success. Here is my Shiny and ggvis code:

plotdata <- reactive({
  this_dataset <- input$dataset
  all = read.csv(paste(this_dataset, ".csv",sep=""), header = T)
  filtered = all[all$year == input$YearSelect,]
  return(filtered)
})

plotdata %>% ggvis(~factor(Var1), ~nums) %>%  
  layer_bars(fill := ~color) %>% 
  add_axis("x", values=mylabels) %>%
  bind_shiny("yearPlot")

When I call

values=mylabels

the function doesn't seem to get called at all and I get the default labels.

The function seems to get called if I write

values=mylabels()

but then I don't know how to pass the default labels as an argument to the function.

I tried

values=mylabels(plotdata$Var1)

but then I get

Error in plotdata$Var1 : object of type 'closure' is not subsettable

If anyone knows a better solution to skip labels, that would be great too. Thanks a lot.