Virtual columns

Sometimes your data source may not be in quite the format you need for your visualisation. For instance, say that you'd like to include a functioning link in a table or tooltip. The solution is to create a "virtual column".

A virtual column can be built from any existing values that exist as a column in the data. So, if your data source contains a column titled Title and a column titled URL you could build a new column named Link using e.g.:

columns:
  - name: 'Link'
    template: '<a href="{{ URL }}">{{ Title }}</a>'

Virtual columns must have name and template properties. The template can make use of string templates, filter functions, and any existing columns in the data.

For example, let's create a new Tooltip contents column:

columns:
  - name: 'Tooltip contents'
    template: '{{ name }}: {{ value | toLocaleString() }}'

This new column is built from existing name and value columns but we pass the value into the filter toLocaleString() to format it. The new column can be referenced elsewhere using Tooltip contents in the same way as you would any pre-existing column in the data. You could have achieved the same result by creating this content directly in the data file but using virtual columns may help to keep that file cleaner.

Defining virtual columns is also a very simple way to rename an exisiting column heading if you need to.