BugFixes to Rotated SignalXY plots

[BrianAtZetica]

Reolution for #3812 had introduced an incorrect final pixel location for rotated SignalXY plots.
This was resolved by a swapping of Xs and Ys coordinates when defining the additional "lastPoint" value.
Note, we could consider renaming Xs and Ys to something like independentValues and dependentValues respectively. Not sure if that will be intuitove to all developers though.

Using the sample code provided in comment on #3812 the fix provided no longer shows a distored line at top of the plot:

Second bug was pre-existing, and became apparent when zoomed into the series to observe the final pixel.
Rotated SignalXY series were performing an incorrect interpolation for the final pixel location (another mix-up of Xs and Ys).

Code to demonstrate this issue is as follows:

        InitializeComponent();

        double[] X = new double[2];
        double[] Y = new double[2];
        X[0] = -10;
        X[1] = 10;
        Y[0] = 1;
        Y[1] = 2;

        var signal = WpfPlot1.Plot.Add.SignalXY(X, Y, color: ScottPlot.Colors.Black);
        signal.Data.Rotated = false;
        var signalrotated = WpfPlot1.Plot.Add.SignalXY(X, Y, color: ScottPlot.Colors.Black);
        signalrotated.Data.Rotated = true;

This plots a normal SignalXY and a rotated SignalXY on the same graph.
Before the bug fix, this becomes distorted when panning the content up or down:

After the bug fix the plot keeps its cross shape while scrolling off the page in any direction:

[BrianAtZetica]

Discovered there is still a problem if Y axis is inverted. Working on this further...