Drawing a horizontal line (hrule) on an rrdtool graph

Installing and Using OpenWrt
1

I would like to draw a green horizontal (threshold) line on an rrdtool graph. I know RRDtool has the HRULE option for it, and I suppose it would have to be incorporated into the corresponding /www/luci-static/resources/statistics/rrdtool/definitions javascript file. What I don't know is whether this option is supported by OpenWrt, and what would it be the correct placement and syntax. I tried some variations of the following without success:

		if ((instances = find_instances('voltage', [ 'input', 'output' ])) != null) {
			definitions.push({
				title: "%H: AC voltages on UPS \"%pi\"",
				vlabel: "Vac",
				y_min: "90",
				y_max: "140",
				alt_autoscale_max: true,
				number_format: "%5.1lfV",
				hrule: "117#00e000:Nominal",
				data: {
					instances: instances,
					options: {
						voltage_output : { color: "00e000", title: "Output voltage", noarea: true, overlay: true },
						voltage_input  : { color: "ffb000", title: "Input voltage", noarea: true, overlay: true }
					}
				}
			});
		}

Note: this could be done for any collectd data, nut is just an example.

How could I achieve this?