Regarding the missing legend, this is related to the way the chart.js code allocates space for a pie chart and it's legends.
There was a thread about it at The size of a pie chart
I think you have to address it with msg.ui_update, slightly shrinking the pie and leaving more space for the legend.
For instance:
[{"id":"86e3b1d66926f4fd","type":"change","z":"d2cca1d7ae48fc5a","name":"","rules":[{"t":"set","p":"ui_update","pt":"msg","to":"{\"chartOptions\":{\"series\":[{\"name\":\"\",\"type\":\"pie\",\"radius\":[\"0%\",\"50%\"],\"center\":[\"50%\",\"50%\"]}]}}","tot":"json"}],"action":"","property":"","from":"","to":"","reg":false,"x":630,"y":320,"wires":[["b17c8b305bc44e13"]]},{"id":"6e31e7ec85c3d61e","type":"inject","z":"d2cca1d7ae48fc5a","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"totalmem\": 15871.977, \"freemem\": 15217.141, \"memusage\": 4.13}","payloadType":"json","x":130,"y":320,"wires":[["ccccc8698ead7945"]]},{"id":"d1ee9925ca9f325e","type":"inject","z":"d2cca1d7ae48fc5a","name":"Clear Dashboard","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[]","payloadType":"jsonata","x":640,"y":360,"wires":[["b17c8b305bc44e13"]]},{"id":"b17c8b305bc44e13","type":"ui-chart","z":"d2cca1d7ae48fc5a","group":"d8b900ab4f0a19c7","name":"Memory Usage","label":"Memory Usage","order":7,"chartType":"pie","category":"","categoryType":"none","xAxisLabel":"","xAxisProperty":"topic","xAxisPropertyType":"msg","xAxisType":"radial","xAxisFormat":"","xAxisFormatType":"auto","xmin":"","xmax":"","yAxisLabel":"","yAxisProperty":"payload","yAxisPropertyType":"msg","ymin":"","ymax":"","bins":10,"action":"append","stackSeries":false,"pointShape":"circle","pointRadius":4,"showLegend":true,"removeOlder":1,"removeOlderUnit":"3600","removeOlderPoints":"","colors":["#0095ff","#ff0000","#ff7f0e","#2ca02c","#a347e1","#d62728","#ff9896","#9467bd","#c5b0d5"],"textColor":["#000000"],"textColorDefault":true,"gridColor":["#e5e5e5"],"gridColorDefault":true,"width":"3","height":"3","className":"","interpolation":"linear","x":840,"y":320,"wires":[[]]},{"id":"b15db461f41e2f6a","type":"split","z":"d2cca1d7ae48fc5a","name":"Split Object","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"topic","property":"payload","x":450,"y":320,"wires":[["86e3b1d66926f4fd"]]},{"id":"ccccc8698ead7945","type":"change","z":"d2cca1d7ae48fc5a","name":"Delete totalmem","rules":[{"t":"set","p":"payload.memusage","pt":"msg","to":"$round((payload.totalmem - payload.freemem),3)","tot":"jsonata"},{"t":"delete","p":"payload.totalmem","pt":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":280,"y":320,"wires":[["b15db461f41e2f6a"]]},{"id":"d8b900ab4f0a19c7","type":"ui-group","name":"NUC","page":"3a65776f23b1f919","width":"12","height":"1","order":1,"showTitle":true,"className":"","visible":"true","disabled":"false","groupType":"default"},{"id":"3a65776f23b1f919","type":"ui-page","name":"Jonas Dashboard","ui":"d45641ed6d39fea1","path":"/page1","icon":"home","layout":"grid","theme":"02a2adba40ee86a4","breakpoints":[{"name":"Default","px":"0","cols":"3"},{"name":"Tablet","px":"576","cols":"6"},{"name":"Small Desktop","px":"768","cols":"9"},{"name":"Desktop","px":"1024","cols":"12"}],"order":1,"className":"","visible":"true","disabled":"false"},{"id":"d45641ed6d39fea1","type":"ui-base","name":"base","path":"/dashboard","appIcon":"","includeClientData":true,"acceptsClientConfig":["ui-notification","ui-control"],"showPathInSidebar":false,"navigationStyle":"default","titleBarStyle":"default","showReconnectNotification":false,"notificationDisplayTime":5,"showDisconnectNotification":false},{"id":"02a2adba40ee86a4","type":"ui-theme","name":"Jonas Theme","colors":{"surface":"#0047ab","primary":"#0094ce","bgPage":"#6d8196","groupBg":"#e0e0e0","groupOutline":"#000000"},"sizes":{"density":"default","pagePadding":"12px","groupGap":"12px","groupBorderRadius":"4px","widgetGap":"12px"}},{"id":"37a489d9f16999d4","type":"global-config","env":[],"modules":{"@flowfuse/node-red-dashboard":"1.30.2"}}]
You might find this thread useful to explore pie chart customisation
Edit: I've been using the word legend in this thread where I should have said label, apologies.
The legend is of course a separate block listing the visible pie sectors, while "memusage" on your example is a label.
Both labels and legends are adversely affected by the chart.js code mis-sizing the gauge in a Node-red widget
