fix sankey trace x/y node fields #545

Job	Run time
Rustfmt	16s
Clippy	3m 27s
semver	50s
Code Coverage	11s
Tests (ubuntu-latest)	2m 6s
Build Book	1m 51s
Tests (macos-latest)	2m 41s
Tests (windows-latest)	3m 39s
Build Wasm Examples (callback-example)	50s
Build Wasm Examples (basic)	56s
Build Examples (3d_charts)	24s
Build Examples (images)	49s
Build Examples (kaleido)	1m 16s
Build Examples (basic_charts)	25s
Build Examples (custom_controls)	24s
Build Examples (ndarray)	28s
Build Examples (shapes)	31s
Build Examples (financial_charts)	25s
Build Examples (subplots)	34s
Build Examples (scientific_charts)	25s
Build Examples (maps)	45s
	23m 13s

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