Format strings for scatter3

[mbchang]

It would be useful to have different format strings in scatter3(), just as there are format strings in plot(). For plotting scatterplots with a large amount of datapoints, for example, I would like to use '.' rather than '+' which is currently the default for scatter3(). Looking at the code it looks like gnuscatter3 doesn't have the format option that gnulplot gets. Can this feature be implemented?