It doesn't make sense to plot y vs f(x1). Here you have several options.
1) You can plot y vs x1 and y vs x2, which can show you how well each variable agrees with y. Cftool is an easy way to play around fits and scatter plots.
2) You can plot y vs x2 vs x1, which is another way to see how variables agree with y. (Cftool does this too)
3) You can plot y vs f(x1,x2) as a scatter plot, which is arguably the most meaningful thing you can plot.
There are a variety of diagnostic plots you can go for if your answer is unsatisfactory. I won't get into those here, check linear regression (assumptions) on wikipedia.