@Gary, there are clues in the error messages that might lead one to the conclusion that scatterPlot is defective. Mouseover of the output error message gives the statement "Skeleton is not a Graphics primitive or directive", suggesting that one of the variables given to Show, possibly scatterPlot, is lacking something. Your last line Show[scatterPlot] gives it's own names as output, showing again that scatterPlot is an empty object. You could write a new line with only scatterPlot and the output would be scatterPlot when it should be a graphics object.