What fires the update () method?

Hi,

I am developing a pretty simple custom visual but I cannot fully understand what fires the update() method. 

I have three data roles and for some strange reason, the update() method is not fired for every field I add to the field pane. It does not seem to be which field I add that causes this issue, it is the order in which I add them. I add a field to the one of the data roles and update() fires. When I add the second field nothing happens. When I then add the third and final field, update() fires again. Could someone help me, I have added the code elements that I think could be relvant below. 

As said, it does not seem to matter which field I populate first, it is always the second field that results in "nothing". 

I have a feeling that it is my getViewModel that is the issue.

capabilities.json

    "dataRoles": [
        {
            "displayName": "KPI Name",
            "name": "category",
            "kind": "Grouping"
        },
        {
            "displayName": "KPI Value",
            "name": "kpimeasure",
            "kind": "Measure"
        },
        {
            "displayName": "Target Value",
            "name": "targetmeasure",
            "kind": "Measure"
        }
    ],



    "dataViewMappings": [ {
            "conditions": [
                {
                    "category": { "max": 1},
                    "kpimeasure": { "max": 1},
                    "targetmeasure": { "max": 1}
                }
            ],
            "categorical": {
                "categories": { 
                    "for": { "in": "category" },
                    "dataReductionAlgorithm": { "top": {} } 
                },
                "values": {
                    "select": [
                        { "bind": { "to": "kpimeasure" } },
                        { "bind": { "to": "targetmeasure" } }
                    ]
                }
            }
        }
    ]

visual.ts

public update(options: VisualUpdateOptions) {
        
        this.settings = VisualSettings.parse<VisualSettings>(options.dataViews[0]); 

        let viewModel = this.getViewModel(options);

        let width = options.viewport.width;
        let height = options.viewport.height;

        this.svg.attr("width", width);
        this.svg.attr("height", height);

        let labelbackground = this.labelBackground
            .attr("width", width)
            .attr("height", height * this.labelSectionPercentage)
            .attr("fill", viewModel.labelBackgroundColor);
        debugger;
        let labeltext = this.labelGroup
            .selectAll(".labelText")
            .data(viewModel.dataPoints, d => (d as DataPoint).label)
            .attr("x", width / 2)
            .attr("y", height * this.labelSectionPercentage / 2)
            .attr("alignment-baseline", "middle")
            .attr("text-anchor", "middle")
            .attr("fill", "white")
            .enter()
            .append("text")
            .classed("labelText", true)
            .text(d => d.label);
    



        this.labelGroup
            .selectAll(".labelText")
            .data(viewModel.dataPoints, d => (d as DataPoint).label)
            .exit()
            .remove();


    }

private getViewModel(options: VisualUpdateOptions): ViewModel {

        let dv = options.dataViews;

        let viewModel: ViewModel = {
            dataPoints: [],
            labelBackgroundColor: "none"
        };

        if (!dv
            || !dv[0]
            || !dv[0].categorical
            || !dv[0].categorical.categories
            || !dv[0].categorical.categories[0].source
            || !dv[0].categorical.values
            || !dv[0].metadata)
            
            {
                console.log(dv);
                return viewModel;
            }

        let view = dv[0].categorical;
        let categories = view.categories[0];
        let values = view.values[0];
        let targets = view.values[1];

        for (let i = 0, len = Math.max(categories.values.length, values.values.length, targets.values.length); i < len; i++) {
            viewModel.dataPoints.push({
                label: <string>categories.values[i],
                value: <number>values.values[i],
                target: <string>targets.values[i]
                
            });
        };
        
        viewModel.labelBackgroundColor = "blue"

        return viewModel;
    }