Function ScatterPlotTooltipContent

Adds tooltip content as a table. The columns of the table are the "label", the value before the mouse cursor, then value after the mouse cursor, and the difference between the two values. The rows of the table are x-values for the first row, and the y-values for the second row. The table has the following form.

series name
           before     after         ∆
x-label     x_tb      x_ta       x_ta - x_tb
y-label     y_tb      y_ta       y_ta - y_tb
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Registers the tooltip-content provider with the ChartContext so that when d3 fires a mouse-over event on a series. The content provider is returns the addTooltipContent function when called. And when called the addTooltipContent function adds the actual tooltip content to the SVG element.