[R] "regression" line of 2 independent variables

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Hi, I would like to a draw a scatterplot of x1 and x2 (plot (x1, x2)), and also 
want to draw a sort of "regression" line across the data points. But x1 and x2 
are just 2 independent variables, so in this case a regression of x1 over x2, or 
vice versa, is not appropriate per se. What would be an appropriate way to do 
this? Thanks for any suggestions.

John