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.
All chart examples passed the displayBlanksAs parameter as 0 instead of 'gap'.
I added a constants EMPTY_AS_GAP, EMPTY_AS_ZERO and EMPTY_AS_SPAN to the
DataSeries and then change all chart samples to use this new constant.
Fixes#1337Closes#1448
oleibman
Adrien Crivelli
Chris Wolcott