MajorGridlineStyle is ignored and always drawn as Solid

[SFSteve]

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Steps to reproduce

1. Create a plotview and add a bottom axis
2. Set the MajorGridlineStyle to anything other than solid or none
3. Run

Platform: Xamarin forms .NET version: standard 2.0

Expected behaviour

The grid lines between the values should be dotted

Actual behaviour

The lines are solid