Jpgraph does not use namespaces. Neither do the Samples members of this project. As a result, Scrutinizer complains about 13 cases of `use PhpOffice\PhpSpreadsheet\Chart\Legend;`, since this theoretically could cause the use of two different Legend classes in the empty namespace. It suggests using a class alias for Legend, which this PR does in every applicable case. It is a regrettable problem, but it is easy to fix.