protected String renderConfig(FacesContext context, Chart component) throws IOException {
        UIComponent facet = component.getFacet("value");
        if (FacetUtils.shouldRenderFacet(facet)) {
            // swap writers
            ResponseWriter originalWriter = context.getResponseWriter();
            FastStringWriter fsw = new FastStringWriter();
            ResponseWriter clonedWriter = originalWriter.cloneWithWriter(fsw);
            context.setResponseWriter(clonedWriter);

            // encode the component
            facet.encodeAll(context);

            // restore the original writer
            context.setResponseWriter(originalWriter);
            return fsw.toString();
        }
        else {
            return component.getValue();
        }
    }
Allow value to be a property or a facet of raw JSON.
primefaces/src/main/java/org/primefaces/component/chart/ChartRenderer.java