How to initialize fixed nodes positions in force-directed layout

[Roy I]

In the documentation (https://github.com/vega/vega/wiki/Data-Transforms),

for property "fixed" of force transform:

"The name of a data set containing nodes whose layout should be fixed. 

This data set may be populated in response to interactions

in order to make nodes stay at a specific coordinate."

This assumes the nodes (with their x,y position) are selected by user interaction (mouse click).

The "force_drag" example in http://vega.github.io/vega-editor/ shows how this works.

Question:

Is there a way to assign certain nodes to have specific fixed x,y positions

at the start of a force-directed layout?