The template node will output msg.payload as it is a js object it can not be used in the mustache template. The output would be [object][object].
Set msg.payload as as json string
e.g
[{"id":"f164ee19.5da43","type":"inject","z":"c791cbc0.84f648","name":"load","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":"1","topic":"","payload":"","payloadType":"date","x":120,"y":320,"wires":[["6a268f39.2a5b1"]]},{"id":"6a268f39.2a5b1","type":"function","z":"c791cbc0.84f648","name":"data","func":"msg = {};\nmsg.payload = `{\n \"$schema\": \"https://vega.github.io/schema/vega/v5.json\",\n \"description\": \"A basic line chart example.\",\n \"width\": 500,\n \"height\": 200,\n \"padding\": 5,\n \n \"signals\": [\n {\n \"name\": \"interpolate\",\n \"value\": \"linear\",\n \"bind\": {\n \"input\": \"select\",\n \"options\": [\n \"basis\",\n \"cardinal\",\n \"catmull-rom\",\n \"linear\",\n \"monotone\",\n \"natural\",\n \"step\",\n \"step-after\",\n \"step-before\"\n ]\n }\n }\n ],\n \n \"data\": [\n {\n \"name\": \"table\",\n \"values\": [\n {\"x\": 0, \"y\": 28, \"c\":0}, {\"x\": 0, \"y\": 20, \"c\":1},\n {\"x\": 1, \"y\": 43, \"c\":0}, {\"x\": 1, \"y\": 35, \"c\":1},\n {\"x\": 2, \"y\": 81, \"c\":0}, {\"x\": 2, \"y\": 10, \"c\":1},\n {\"x\": 3, \"y\": 19, \"c\":0}, {\"x\": 3, \"y\": 15, \"c\":1},\n {\"x\": 4, \"y\": 52, \"c\":0}, {\"x\": 4, \"y\": 48, \"c\":1},\n {\"x\": 5, \"y\": 24, \"c\":0}, {\"x\": 5, \"y\": 28, \"c\":1},\n {\"x\": 6, \"y\": 87, \"c\":0}, {\"x\": 6, \"y\": 66, \"c\":1},\n {\"x\": 7, \"y\": 17, \"c\":0}, {\"x\": 7, \"y\": 27, \"c\":1},\n {\"x\": 8, \"y\": 68, \"c\":0}, {\"x\": 8, \"y\": 16, \"c\":1},\n {\"x\": 9, \"y\": 49, \"c\":0}, {\"x\": 9, \"y\": 25, \"c\":1}\n ]\n }\n ],\n \n \"scales\": [\n {\n \"name\": \"x\",\n \"type\": \"point\",\n \"range\": \"width\",\n \"domain\": {\"data\": \"table\", \"field\": \"x\"}\n },\n {\n \"name\": \"y\",\n \"type\": \"linear\",\n \"range\": \"height\",\n \"nice\": true,\n \"zero\": true,\n \"domain\": {\"data\": \"table\", \"field\": \"y\"}\n },\n {\n \"name\": \"color\",\n \"type\": \"ordinal\",\n \"range\": \"category\",\n \"domain\": {\"data\": \"table\", \"field\": \"c\"}\n }\n ],\n \n \"axes\": [\n {\"orient\": \"bottom\", \"scale\": \"x\"},\n {\"orient\": \"left\", \"scale\": \"y\"}\n ],\n \n \"marks\": [\n {\n \"type\": \"group\",\n \"from\": {\n \"facet\": {\n \"name\": \"series\",\n \"data\": \"table\",\n \"groupby\": \"c\"\n }\n },\n \"marks\": [\n {\n \"type\": \"line\",\n \"from\": {\"data\": \"series\"},\n \"encode\": {\n \"enter\": {\n \"x\": {\"scale\": \"x\", \"field\": \"x\"},\n \"y\": {\"scale\": \"y\", \"field\": \"y\"},\n \"stroke\": {\"scale\": \"color\", \"field\": \"c\"},\n \"strokeWidth\": {\"value\": 2}\n },\n \"update\": {\n \"interpolate\": {\"signal\": \"interpolate\"},\n \"strokeOpacity\": {\"value\": 1}\n },\n \"hover\": {\n \"strokeOpacity\": {\"value\": 0.5}\n }\n }\n }\n ]\n }\n ]\n \n }\n}`\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":260,"y":320,"wires":[["97e59b4b.7e8978"]]},{"id":"97e59b4b.7e8978","type":"template","z":"c791cbc0.84f648","name":"","field":"payload","fieldType":"msg","format":"handlebars","syntax":"mustache","template":"<head>\n <meta charset=\"utf-8\">\n <script src=\"https://cdn.jsdelivr.net/npm/vega@5\"></script>\n <script src=\"https://cdn.jsdelivr.net/npm/vega-embed@6\"></script>\n</head>\n<body>\n <div id=\"vis\" style=\"height:449px\">\n </div>\n</body>\n<script>\n const spec = {{{payload}}}\n \n \tvegaEmbed(\"#vis\", spec)\n \t// result.view provides access to the Vega View API\n .then(result => console.log(result))\n .catch(console.warn);\n</script>\n\n","output":"str","x":410,"y":320,"wires":[["73230270.560b6c"]]},{"id":"73230270.560b6c","type":"debug","z":"c791cbc0.84f648","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":600,"y":360,"wires":[]}]
