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Category Archives: vega-lite

A Tale of Two Charting Paradigms: Vega-Lite vs R+ggplot2

This post comes hot off the heels of the [nigh-feature-complete release of `vegalite`](http://rud.is/b/2016/02/27/create-vega-lite-specs-widgets-with-the-vegalite-package/) (virtually all the components of Vega-Lite are now implemented and just need real-world user testing). I’ve had a few and seen a few questions about “why Vega-Lite”? I _think_ my previous post gave some good answers to “why”. However, Vega-Lite and Vega provide different ways to think about composing statistical graphs than folks seem to be used to (which is part of the “why?”).

Vega-Lite attempts to simplify the way charts are specified (i.e. the way you create a “spec”) in Vega. Vega-proper is rich and complex. You interleave data, operations on data, chart aesthetics and chart element interactions all in one giant JSON file. Vega-Lite 1.0 is definitely more limited than Vega-proper and even when it does add more interactivity (like “brushing”) it will _still_ be more limited, _on purpose_. The reduction in complexity makes it more accessible to both humans and apps, especially apps that don’t grok the Grammar of Graphics (GoG) well.

Even though `ggplot2` lets you mix and match statistical operations on data, I’m going to demonstrate the difference in paradigms/idioms through a single chart. I grabbed the [FRED data on historical WTI crude oil prices](https://research.stlouisfed.org/fred2/series/DCOILWTICO) and will show a chart that displays the minimum monthly price per-decade for a barrel of this cancerous, greed-inducing, global-conflict-generating, atmosphere-destroying black gold.

The data consists of records of daily prices (USD) for this commodity. That means we have to:

1. compute the decade
2. compute the month
3. determine the minimum price by month and decade
4. plot the values

The goal of each idiom is to provide a way to reproduce and communicate the “research”.

Here’s the idiomatic way of doing this with Vega-Lite:

library(vegalite)
library(quantmod)
library(dplyr)
 
getSymbols("DCOILWTICO", src="FRED")
 
data_frame(date=index(DCOILWTICO),
           value=coredata(DCOILWTICO)[,1]) %>%
  mutate(decade=sprintf("%s0", substring(date, 1, 3))) -> oil
 
# i created a CSV and moved the file to my server for easier embedding but
# could just have easily embedded the data in the spec.
# remember, you can pipe a vegalite object to embed_spec() to
# get javascript embed code.
 
vegalite() %>%
  add_data("http://rud.is/dl/crude.csv") %>%
  encode_x("date", "temporal") %>%
  encode_y("value", "quantitative", aggregate="min") %>%
  encode_color("decade", "nominal") %>%
  timeunit_x("month") %>%
  axis_y(title="", format="$3d") %>%
  axis_x(labelAngle=45, labelAlign="left", 
         title="Min price for Crude Oil (WTI) by month/decade, 1986-present") %>%
  mark_tick(thickness=3) %>%
  legend_color(title="Decade", orient="left")

Here’s the “spec” that creates (wordpress was having issues with it, hence the gist embed):

And, here’s the resulting visualization:

The grouping and aggregation operations operate in-chart-craft-situ. You have to carefully, visually parse either the spec or the R code that creates the spec to really grasp what’s going on. A different way of looking at this is that you embed everything you need to reproduce the transformations and visual encodings in a single, simple JSON file.

Here’s what I believe to be the modern, idiomatic way to do this in R + `ggplot2`:

library(ggplot2)
library(quantmod)
library(dplyr)
 
getSymbols("DCOILWTICO", src="FRED")
 
data_frame(date=index(DCOILWTICO),
           value=coredata(DCOILWTICO)[,1]) %>%
  mutate(decade=sprintf("%s0", substring(date, 1, 3)),
         month=factor(format(as.Date(date), "%B"),
                      levels=month.name)) -> oil
 
filter(oil, !is.na(value)) %>%
  group_by(decade, month) %>%
  summarise(value=min(value)) %>%
  ungroup() -> oil_summary
 
ggplot(oil_summary, aes(x=month, y=value, group=decade)) +
  geom_point(aes(color=decade), shape=95, size=8) +
  scale_y_continuous(labels=scales::dollar) +
  scale_color_manual(name="Decade", 
                     values=c("#d42a2f", "#fd7f28", "#339f34", "#d42a2f")) +
  labs(x="Min price for Crude Oil (WTI) by month/decade, 1986-present", y=NULL) +
  theme_bw() +
  theme(axis.text.x=element_text(angle=-45, hjust=0)) +
  theme(legend.position="left") +
  theme(legend.key=element_blank()) +
  theme(plot.margin=grid::unit(rep(1, 4), "cm"))

(To stave off some comments, yes I do know you can be Vega-like and compute with arbitrary functions within ggplot2. This was meant to show what I’ve seen to be the modern, recommended idiom.)

You really don’t even need to know R (for the most part) to grok what’s going on. Data is acquired and transformed and we map that into the plot. Yes, you can do the same thing with Vega[-Lite] (i.e. munge the data ahead of time and just churn out marks) but _you’re not encouraged to_. The power of the Vega paradigm is that you _do blend data and operations together_ and they _stay together_.

To make the R+ggplot2 code reproducible the entirety of the script has to be shipped. It’s really the same as shipping the Vega[-Lite] spec, though since you need to reproduce either the JSON or the R code in environments that support the code (R just happens to support both ggplot2 & Vega-Lite now :-).

I like the latter approach but can appreciate both (otherwise I wouldn’t have written the `vegalite` package). I also think Vega-Lite will catch on more than Vega-proper did (though Vega itself is in use and you use under the covers whenever you use `ggvis`). If Vega-Lite does nothing more than improve visualization literacy—you _must_ understand core vis terms to use it—and foster the notion for the need for serialization, reproduction and sharing of basic statistical charts, it will have been an amazing success in my book.

Create Vega-Lite specs & widgets with the vegalite package

[Vega-Lite](http://vega.github.io/vega-lite/) 1.0 was [released this past week](https://medium.com/@uwdata/introducing-vega-lite-438f9215f09e#.yfkl0tp1c). I had been meaning to play with it for a while but I’ve been burned before by working with unstable APIs and was waiting for this to bake to a stable release. Thankfully, there were no new shows in the Fire TV, Apple TV or Netflix queues, enabling some fast-paced nocturnal coding to make an [R `htmlwidget`s interface](https://github.com/hrbrmstr/vegalite) to the Vega-Lite code before the week was out.

What is “Vega” and why “-Lite”? [Vega](http://vega.github.io/) is _”a full declarative visualization grammar, suitable for expressive custom interactive visualization design and programmatic generation.”_ Vega-Lite _”provides a higher-level grammar for visual analysis, comparable to ggplot or Tableau, that generates complete Vega specifications.”_ Vega-Lite compiles to Vega and is more compact and accessible than Vega (IMO). Both are just JSON data files with a particular schema that let you encode the data, encodings and aesthetics for statistical charts.

Even I don’t like to write JSON by hand and I can’t imagine anyone really wanting to do that. I see Vega and Vega-Lite as amazing ways to serialize statistical charts from ggplot2 or even Tableau (or any Grammar of Graphics-friendly creation tool) and to pass around for use in other programs—like [Voyager](http://vega.github.io/voyager/) or [Pole★](http://vega.github.io/polestar/)—or directly on the web. It is “glued” to D3 (given the way data transformations are encoded and colors are specified) but it’s a pretty weak glue and one could make a Vega or Vega-Lite spec render to anything given some elbow grease.

But, enough words! Here’s how to make a simple Vega-Lite bar chart using `vegalite`:

# devtools::install_github("hrbrmstr/vegalite")
library(vegalite)
 
dat <- jsonlite::fromJSON('[
    {"a": "A","b": 28}, {"a": "B","b": 55}, {"a": "C","b": 43},
    {"a": "D","b": 91}, {"a": "E","b": 81}, {"a": "F","b": 53},
    {"a": "G","b": 19}, {"a": "H","b": 87}, {"a": "I","b": 52}
  ]')
 
vegalite() %>% 
  add_data(dat) %>%
  encode_x("a", "ordinal") %>%
  encode_y("b", "quantitative") %>%
  mark_bar()

Note that bar graph you see above is _not_ a PNG file or `iframe`d widget. If you `view-source:` you’ll see that I was able to take the Vega-Lite generated spec for that widget code (done by piping the widget to `to_spec()`) and just insert it into this post via:

<style media="screen">.wpvegadiv { display:inline-block; margin:auto }</style>
 
<center><div id="vlvis1" class="wpvegadiv"></div></center>
 
<script>
var spec1 = JSON.parse('{"description":"","data":{"values":[{"a":"A","b":28},{"a":"B","b":55},{"a":"C","b":43},{"a":"D","b":91},{"a":"E","b":81},{"a":"F","b":53},{"a":"G","b":19},{"a":"H","b":87},{"a":"I","b":52}]},"mark":"bar","encoding":{"x":{"field":"a","type":"ordinal"},"y":{"field":"b","type":"quantitative"}},"config":[],"embed":{"renderer":"svg","actions":{"export":false,"source":false,"editor":false}}} ');
 
var embedSpec = { "mode": "vega-lite", "spec": spec1, "renderer": spec1.embed.renderer, "actions": spec1.embed.actions };
 
vg.embed("#vlvis1", embedSpec, function(error, result) {});
</script>

I did have have all the necessary js libs pre-loaded like you see [in this example](http://vega.github.io/vega-lite/tutorials/getting_started.html#embed). You can use the `embed_spec()` function to generate most of that for you, too.

This means you can use R to gather, clean, tidy and analyze data. Then, generate a visualization based on that data with `vegalite`. _Then_ generate a lightweight JSON spec from it and easily embed it anywhere without having to rig up a way to get a widget working or ship giant R markdown created files (like [this one](http://rud.is/projects/vegalite01.html) which has many full `vegalite` widgets on it).

One powerful feature of Vega/Vega-Lite is that the data does not have to be embedded in the spec.

Take this streamgraph visualization about unemployment levels across various industries over time:

vegalite() %>%
  cell_size(500, 300) %>%
  add_data("https://vega.github.io/vega-editor/app/data/unemployment-across-industries.json") %>%
  encode_x("date", "temporal") %>%
  encode_y("count", "quantitative", aggregate="sum") %>%
  encode_color("series", "nominal") %>%
  scale_color_nominal(range="category20b") %>%
  timeunit_x("yearmonth") %>%
  scale_x_time(nice="month") %>%
  axis_x(axisWidth=0, format="%Y", labelAngle=0) %>%
  mark_area(interpolate="basis", stack="center")

The URL you see in the R code is placed into the JSON spec. That means whenever that data changes and the visualization is refreshed, you see updated content without going back to R (or js code).

Now, dynamically-created visualizations are great, but what if you want to actually let your viewers have a copy of it? With Vega/Vega-Lite, you don’t need to resort to hackish bookmarklets, just change a configuration option to enable an export link:

vegalite(export=TRUE) %>%
  add_data("https://vega.github.io/vega-editor/app/data/seattle-weather.csv") %>%
  encode_x("date", "temporal") %>%
  encode_y("*", "quantitative", aggregate="count") %>%
  encode_color("weather", "nominal") %>%
  scale_color_nominal(domain=c("sun","fog","drizzle","rain","snow"),
                      range=c("#e7ba52","#c7c7c7","#aec7e8","#1f77b4","#9467bd")) %>%
  timeunit_x("month") %>%
  axis_x(title="Month") %>% 
  mark_bar()

(You can style/place that link however/wherever you want. It’s a simple classed `

`.)

If you choose a `canvas` renderer, the “export” option will be PNG vs SVG.

The package is nearly (~98%) feature complete to the 1.0 Vega-Lite standard. There are some tedious bits from the Vega-Lite spec remaining to be encoded. I’ve transcribed much of the Vega-Lite documentation to R function & package documentation with links back to the Vega-Lite sources if you need more detail.

I’m hoping to be able to code up an “`as_spec()`” function to enable quick conversion of ggplot2-created graphics to Vega-Lite (and support converting a ggplot2 object to a Vega-Lite spec in `to_spec()`) but that won’t be for a while unless someone wants to jump on board and implement an Vega expression creator/parser in R for me :-)

You can work with the current code [on github](https://github.com/hrbrmstr/vegalite) and/or jump on board to help with package development or file an issue with an idea or a bug. Please note that this package is under _heavy development_ and the function interface is very likely to change as I and others work with it and develop more streamlined ways to handle the encodings. Check back to the github repo often to find out what’s different (there will be a `NEWS` file posted soon and maintained as well).