Default conversion

R objects provided as data for D3 visualizations are converted to JSON using the jsonlite::toJSON() function, and use the same default serialization behavior as Shiny and htmlwidgets.

This corresponds to the following call to jsonlite::toJSON():

Data frames

Data frames are serialized with a columns orientation as that is a more compact over the wire representation than rows orientation. When the data frame gets to the client r2d3 calls the HTMLWidgets.dataframeToD3() method to transform the data to a D3-friendly rows orientation.

Here is an example of the JSON columns-based representation of an R data frame:

{
  "Sepal.Length": [5.1, 4.9, 4.7],
  "Sepal.Width": [3.5, 3, 3.2],
  "Petal.Length": [1.4, 1.4, 1.3],
  "Petal.Width": [0.2, 0.2, 0.2],
  "Species": ["setosa", "setosa", "setosa"]
} 

After we apply HTMLWidgets.dataframeToD3(), it will become:

[
  {
    "Sepal.Length": 5.1,
    "Sepal.Width": 3.5,
    "Petal.Length": 1.4,
    "Petal.Width": 0.2,
    "Species": "setosa"
  },
  {
    "Sepal.Length": 4.9,
    "Sepal.Width": 3,
    "Petal.Length": 1.4,
    "Petal.Width": 0.2,
    "Species": "setosa"
  },
  {
    "Sepal.Length": 4.7,
    "Sepal.Width": 3.2,
    "Petal.Length": 1.3,
    "Petal.Width": 0.2,
    "Species": "setosa"
  }
] 

Custom conversion

If you don’t like the default JSON conversion provided for data, you can write your own function that uses jsonlite to perform a custom conversion. For example:

data_to_json <- function(data) {
  jsonlite::toJSON(data, dataframe = "rows", auto_unbox = FALSE, rownames = TRUE)
}

r2d3(data = data_to_json(x), script = "barchart.js")

When a value returned from jsonlite::toJSON() is passed as the data argument r2d3 won’t attempt any additional conversion or transformation of the value.

S3 conversion method

You can implement the as_d3_data() S3 method to provide custom converters for any R class. For example, you could create an S3 method for the igraph class to convert data into a JSON that is optimized for rendering by a D3 network visualization: