As of version 4.2, charts support data in {x-axis, series1-value, series2-value,..} format, which matches natural structure of table in most cases. However charts do not support data in {x-axis, series-name, value} format, which was the format used prior to version 4.0. It's better to support both of these formats, with some ui selection given to the user to select between them.
Diff:
Implemented with https://github.com/phpmyadmin/phpmyadmin/commit/cc0fa75cce7b20b5dec45429a76f21f835ace7a4