Highcharts X axis not showing properly

I want to show the distribution of windturbine's events using Highcharts.

My X-axis won't display its values entirely well. I found that when I have only one event defined for one year, it mucks up the vizualisation.

However when I have multiple events on several years, it goes smooth. I don't know if the code misses one option or if I should use another function to display my results.

Below is the code and the associated results.

library(highcharter)
library(dplyr)
library(highcharter)
    mpgg_EX <- data.frame("Country" = c("Belux","Belux","Belux","Belux","Belux"), "WT_Manufacturer" = c("bubble","bubble","bubble","bubble","fever"), "Start_Event_Date" = c("2013", "2014","2015","2016","2013"),"BLA"=c(0,0,0,0,0),"ELEC"=c(1,0,0,0,0),"MECA"=c(1,1,1,1,1),"OTH"=c(0,0,0,0,0),"PWEL"=c(0,1,0,0,0),"VAR"=c(1,0,0,0,0),stringsAsFactors = FALSE)

       categories_grouped_EX <- mpgg_EX %>% 
        group_by(name = WT_Manufacturer) %>% 
        do(categories = .$Start_Event_Date) %>% 
        list_parse()
    categories_grouped_EX<- lapply(categories_grouped_EX, function(x) x[!is.na(x)])

      hc_indispo_EX <- highchart() %>%
        hc_chart(type = "column") %>%
        hc_colors("#fc5421") %>%   
        hc_title(text = "Unavailability Manufacturer Example") %>%
        hc_xAxis(title = "Suppliers"
                ,type = "category"
                ,categories = categories_grouped_EX
                 ) %>%
        hc_legend(layout = "vertical", verticalAlign = "bottom",align = "right", valueDecimals = 0, reversed = TRUE) %>% 

         hc_plotOptions(
          series = list(
            cropThreshold = 1000,
            stacking = "normal",
            borderWidth = 0,
            grouping = TRUE,
            dataLabels = list(enabled = FALSE)
          )
        ) %>%
            hc_add_series(
          data = mpgg_EX,
          type = "column",
          name = "MECA",
          hcaes(x = Start_Event_Date,
                y = MECA)
         ) %>%
        hc_tooltip(formatter = JS("function(){
           var point = this.point;
           s = '<b>' + this.series.name + '</b>' + ' :' + Highcharts.numberFormat(this.y,0)  + '<br/>';
            s += '<b>' + 'Total : ' + '</b>' + Highcharts.numberFormat(point.stackTotal,0)  + '<br/>';
           return (s)}"
           ))

      hc_indispo_EX