I'm not sure if there is a more fundamental way to do this, but you can extract the data from the plot.
load carsmall
Year = categorical(Model_Year);
tbl = table(MPG,Weight,Year);
mdl = fitlm(tbl,'MPG ~ Year + Weight^2');
figure
h = plotAdded(mdl);
% h has three handles. In this case, the first one has the data.
% (In your case, you may need to look at h to figure out which handle is correct.)
xData = h(1).XData;
% Show a few values
xData(1:5)
