The graph results are give below after playing around with the value of K.
In general, the plot of kNN smoothing has many discontinuities.
For K=1, the kNN smoothing line goes passing all the data points, therefore the sum of square error is zero.
The plot is the most rough.
When K=5 (all the data point), we get only one horizontal line as the average of all data.
Between the two extremes, we can find adjust the value of K as parameter to adjust the smoothing plot.
Among K=2, K=3 and K=4, we obtain K=4 have the smallest sum of squared error (SSE) = Σni=1(Xi-X)2.