Is it possible to get the data table column names in Vega-lite as an array? - pandas

Question

Is there a Vega/Vega-lite equivalent of pandas' df.columns?
I would like to get an array with all column names in Vega-lite. For this dataset:
{"data": { "values" : [{"a":4, "b":5},{"a":6, "b":7}] }}
I would like to get an array ["a","b"]. This would be extremely helpful when using the fold transform with large datasets.

Answer 1

Here is one way to get a Vega dataset fieldnames using Vega API.
In Vega spec, add signal declaration, e.g.:
"signals":[
{"name": "signal_data_table_fieldnames"
}
In javacript, use Vega view API methods signal and data to retrieve the first record of the dataset table and obtain the fieldnames as an array with javascript Object.keys()
When using the view.signal, the Vega API method can both get and set the internal Vega signal value. The updated signal value can be used anywhere within the Vega spec.
Vega API docs for signal and data methods:
https://vega.github.io/vega/docs/signals/
https://vega.github.io/vega/docs/api/view/#signals
https://vega.github.io/vega/docs/api/view/#data-and-scales
Example code using
https://vega.github.io/vega/usage/
with bar chart example
https://vega.github.io/vega/examples/bar-chart/ :
var view;
fetch('https://vega.github.io/vega/examples/bar-chart.vg.json')
.then(res => res.json())
.then(spec => render(spec))
.catch(err => console.error(err));
function render(spec) {
view = new vega.View(vega.parse(spec), {
renderer: 'canvas', // renderer (canvas or svg)
container: '#view', // parent DOM container
hover: true // enable hover processing
});
return view.runAsync();
}
// Vega API
view.signal("signal_data_table_fieldnames", Object.keys(view.data("table")[0]));
console.log(view.signal("signal_data_table_fieldnames"));