I am reopening this because I found out how to do this:

graph.echarts.on('graphRoam', function(e) {
  console.log(graph.echarts.getOption().series[0].zoom);
});

Hello guys, I am interested in contributing to solve this issue. Potential applicant for GSOC.

Spent some time to understand the way how code works and, honestly, could not find the implementation for this (mentioned by @nemesifier):

graph.echarts.on('graphRoam', function(e) {
console.log(graph.echarts.getOption().series[0].zoom);
});

However, I have noticed that in the example with netjsonmap.html, there is an attribute called zoom:

// set map initial state.
        mapOptions: {
          center: [46.86764405052012, 19.675998687744144],
          zoom: 5,
          nodeConfig: {
            label: {
              offset: [0, -10],
            },
          },
        },

So I think, it is possible to fix this issue by manipulating this zoom attribute in core files of netjsongraph.js. I wanted to make sure if this is the correct way of manipulations with zoom (code from src/js/netjsongraph.render.js):

self.leaflet.on("zoomend", () => {
        if (self.leaflet.getZoom() < self.config.disableClusteringAtLevel) {

Also, is the initial state for map from netjsonmap.html saved as config in NetJSONGraph class in netjsongraph.js:

class NetJSONGraph { 
  /**
   * @constructor
   *
   * @param {string} JSONParam - The NetJSON file parameter.
   * @param {Object} [config={}] - Configuration options for the graph.
   */
  constructor(JSONParam, config = {}) {