if you only want to update one trace, you still have to provide an array containing an array for that one trace: update = {'x': [[x]], 'y': [[y]]};

OMG, this saved my bacon, thanks! I've been struggling for a half day(!) trying to update a couple of traces in a plotly polar plot on mouse-mnove and, although my overall syntax was spot on, I simply did not know that DOUBLE square brackets would be needed round such values. Now I know how it's done, I can get on with the rest of the coding needed. Thanks again!