This PR resolves some minor and some major rendering artefacts that were occurrign in Rotated SignalXY plots.
This incorporates changes made in a prevfious PR (#4102 ) that also attempted to refactor the plottable type to remove confusion between X and Y data types and X and Y axis directions.

The main artefact was best illustrated when plotting a rotated signal on inverted axes

        var size = 500;
        var cycles = 1;
        double[] xs = Generate.Consecutive(size);
        double[] ys = Generate.Sin(count: xs.Length, oscillations: cycles);

        var sig1 = WpfPlot1.Plot.Add.SignalXY(xs, ys);
        sig1.Data.Rotated = true;
        WpfPlot1.Plot.Axes.InvertX();
        WpfPlot1.Plot.Axes.InvertY();

After this PR the same code produces a smooth sinusoid:

The other artefacts demonstrated in #4102 are also resolved.

The PR also:

• implements Rotated plots in the generic data type variant of SIgnal XY
• Introduces an XOffset parameter (this is an offset along the uniform dimension of the SignalXY).