add example of subplots with multiple traces per subplot #560

Job	Run time
Clippy	3m 10s
semver	42s
Rustfmt	14s
Code Coverage	9s
Tests (ubuntu-latest)	2m 0s
Tests (windows-latest)	3m 34s
Build Book	1m 38s
Tests (macos-latest)	2m 45s
Build Wasm Examples (basic)	49s
Build Wasm Examples (callback-example)	49s
Build Examples (3d_charts)	25s
Build Examples (custom_controls)	23s
Build Examples (basic_charts)	26s
Build Examples (financial_charts)	25s
Build Examples (ndarray)	26s
Build Examples (scientific_charts)	27s
Build Examples (maps)	46s
Build Examples (customization)	31s
Build Examples (shapes)	31s
Build Examples (kaleido)	1m 17s
Build Examples (images)	39s
Build Examples (subplots)	24s
Build Examples (themes)	25s
	22m 55s

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