Unit testing now results in 100% coverage for Axis and Properties. All the properties in methods in Gridlines were more or less duplicated in Axis, and these duplications are moved to the common ancestor Properties. So, there isn't anything left in Gridlines. PhpSpreadsheet Chart is now over 85% covered (it was below 35% until recently).
Properties are in many cases set to default to null/null-string, rather than the default values they receive from Excel, and are not written to Xml if unchanged. This is consistent with how Excel behaves. A new property `crossBetween` is added to Axis, and, with support for that added to Xlsx Reader and Writer, some minor Sample peculiarities are corrected, in particular, the charts were sometimes slightly truncated on the left and right edges.
97381d4307