vegaliteerror

error

processing file: vegalitetest.Rmd
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  ordinary text without R code

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label: setup (with options) 
List of 1
 $ include: logi FALSE

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label: vegalite

Quitting from lines 27-42 (vegalitetest.Rmd) 
Error in vis$data[[name]] <- data : 
  invalid type/length (closure/0) in vector allocation
Calls: <Anonymous> ... eval -> _fseq -> freduce -> <Anonymous> -> add_data
Execution halted

SessionInfo

> sessionInfo()
R version 3.2.3 (2015-12-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United Kingdom.1252   
[3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C                           
[5] LC_TIME=English_United Kingdom.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.9.6    vegalite_0.5.1.9000

loaded via a namespace (and not attached):
 [1] htmlwidgets_0.6   clipr_0.2.0       magrittr_1.5      htmltools_0.3     tools_3.2.3       yaml_2.1.13      
 [7] rmarkdown_0.9.5.1 knitr_1.12.3      jsonlite_0.9.19   digest_0.6.9      chron_2.3-47     

vegalitetest.Rmd

---
title: "Untitled"
author: "Steph Locke"
date: "4 March 2016"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
# Not on CRAN
if(!require(devtools)) install.packages("devtools")
if(!require(vegalite)) devtools::install_github("hrbrmstr/vegalite")
if(!require(ggvis)) install.packages("ggvis")
if(!require(data.table)) install.packages("data.table")

library(data.table)

seatbelts<-data.table(date=seq(as.Date("1969-01-01")
                               ,as.Date("1984-12-01")
                               ,by = "month")
                      , Seatbelts)[
                        ,year:=year(date)
                      ]
```

```{r vegalite}
library(vegalite)
library(data.table)

seatbelts<-data.table(date=seq(as.Date("1969-01-01")
                               ,as.Date("1984-12-01")
                               ,by = "month")
                      , Seatbelts)[
                        ,year:=year(date)
                      ]
vegalite() %>%
  add_data(seatbelts) %>%
  encode_x("PetrolPrice","quantitative") %>%
  encode_y("DriversKilled", "quantitative") %>%
  mark_point()

```

```{r ggvis}
library(ggvis)
seatbelts %>%
  ggvis(~as.factor(date), ~DriversKilled) %>%
  layer_bars()
```