The tag for indicating the dataset is <cvData>
Example
subdataset
<subDataset name="Dataset1" uuid="03d50d7f-1b96-486a-ac64-7d2c6e440433">
<queryString>
<![CDATA[select count(*) t, shipcountry, shipcity from orders group by shipcountry, shipcity order by shipcountry, shipcity]]>
</queryString>
<field name="SHIPCOUNTRY" class="java.lang.String"/>
<field name="T" class="java.lang.Long"/>
<field name="SHIPCITY" class="java.lang.String"/>
</subDataset>
component
<cvc:customvisualization xmlns:cvc="http://www.jaspersoft.com/cvcomponent" xsi:schemaLocation="http://www.jaspersoft.com/cvcomponent http://www.jaspersoft.com/cvcomponent/component.xsd" evaluationTime="Report" onErrorType="Icon">
<cvc:itemProperty name="script" value="d3_zoomable_circle_packing.min.js"/>
<cvc:itemProperty name="css" value="d3_zoomable_circle_packing.css"/>
<cvc:cvData>
<dataset>
<datasetRun subDataset="Dataset1" uuid="bd23d50f-2149-4985-a0ac-883505172688">
<parametersMapExpression><![CDATA[$P{REPORT_PARAMETERS_MAP}]]></parametersMapExpression>
<connectionExpression><![CDATA[$P{REPORT_CONNECTION}]]></connectionExpression>
</datasetRun>
</dataset>
<cvc:item>
<cvc:itemProperty name="category">
<valueExpression><![CDATA[$F{SHIPCOUNTRY}]]></valueExpression>
</cvc:itemProperty>
<cvc:itemProperty name="subcategory">
<valueExpression><![CDATA[$F{SHIPCITY}]]></valueExpression>
</cvc:itemProperty>
<cvc:itemProperty name="value">
<valueExpression><![CDATA[$F{T}]]></valueExpression>
</cvc:itemProperty>
</cvc:item>
</cvc:cvData>
</cvc:customvisualization>
EDIT: As @dada67 comment it does not seem to work properly, I have also test it with sample d3_zoomable_cricle_packing.jrxml
without success, this is the bug issue
The work around, create a subreport!