Chart: add `show_legend` toggle, document it, and add e2e test #2349

Job	Run time
docker_build (linux/arm64, minimal)	6m 13s
windows_test	2m 42s
docker_build (linux/arm64, duckdb)	6m 44s
docker_build (linux/amd64, minimal)	5m 37s
test (mssql, mssql, mssql://root:Password123!@127.0.0.1/sqlpage)	3m 52s
test (oracle, oracle, Driver=Oracle 21 ODBC driver;Dbq=//127.0.0.1:1521/FREEPDB1;Uid=root;Pwd=Pas...	4m 42s
docker_build (linux/arm/v7, minimal)	5m 35s
compile_and_lint	5m 11s
docker_build (linux/amd64, duckdb)	5m 26s
test (mysql, mysql, mysql://root:Password123!@127.0.0.1/sqlpage)	3m 24s
test (postgres, postgres, postgres://root:Password123!@127.0.0.1/sqlpage)	3m 32s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	3m 54s
docker_push	0s
	56m 52s

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