* Move Gridlines from Chart to Axis
This could, I hope, be my last major change to Chart for a while. When I first noticed this problem, I thought it would be a breaking change. However, although this change establishes some deprecations, I don't think it breaks anything. Major and minor gridlines had only been settable by the Chart constructor. This PR moves them where they belong, to Axis (eexisting Chart constructor code will still work). This allows them to be specified from both X and Y axis.
Chart is now entirely covered except for 2 statements, one deprecated and one that I just can't figure out. 99.71% for Charts, 88.96% overall. All references to the Chart directory in Phpstan baseline are eliminated.
* Minor Fixes, Unit Tests
Line style color type should default to null not prstClr.
Chart X-axis and Y-axis should alway be Axis, never null.
Add some unit tests.
* More Tests, Some Improvements
Make it easier to change line styles, adding an alternate method besides a setter function with at least a dozen parameters.
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