add another example of subplots with multiple traces per subplot #561

Job	Run time
Rustfmt	14s
Code Coverage	11s
Tests (ubuntu-latest)	1m 55s
semver	42s
Tests (windows-latest)	3m 46s
Tests (macos-latest)	3m 11s
Build Book	1m 33s
Clippy	3m 7s
Build Wasm Examples (basic)	47s
Build Examples (financial_charts)	25s
Build Examples (3d_charts)	24s
Build Wasm Examples (callback-example)	50s
Build Examples (images)	37s
Build Examples (custom_controls)	24s
Build Examples (ndarray)	30s
Build Examples (shapes)	24s
Build Examples (subplots)	24s
Build Examples (scientific_charts)	23s
Build Examples (maps)	35s
Build Examples (basic_charts)	33s
Build Examples (themes)	26s
Build Examples (customization)	24s
Build Examples (kaleido)	1m 12s
	22m 57s

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