Probably the only way to do that is to use the 'SamplePoints' name-value pair, and smooth all but the beginning and end points —
x = 1:25;
y = randn(size(x));
ys = smoothdata(y(2:end-1), 'SamplePoints',x(2:end-1));
figure
scatter(x, y, 'p')
hold on
plot(x, [y(1) ys y(end)], '-r')
hold off
grid
.
