ScatterPlot: Add OffsetX and OffsetY

[swharden]

Feature description: Scatter plots should have X and Y offsets to allow "shifting" display of the data without requiring modification of the values in the arrays

Code example:

var sp = plt.AddScatter(xs, ys);
sp.OffsetX = 123;
sp.OffsetY = 456;

This idea was raised in #1002 and implemented in #1111 (not yet merged)