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It is because you only have 7 labels on the x axis.
If you add more like so by changing labels from:
labels: [1,2,3,4,5,6,7],
to:
labels: [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]
It works fine.
Remember you are asking for a max of 20 but there are only 7 so it can only display 7.
Source:stackexchange.com