@ezsolve,
Axis labels are shown at every interval which is auto-calculated based on parameters like axis minimum, axis maximum, etc. In your case, the auto calculated interval is 2 months and hence ‘Dec 2019’ is not shown. You can set the interval to 1 month to show the label in Dec 2019 by using interval and intervalType properties as shown in the code-snippet below.
axisX: {
interval: 1,
intervalType: "month"
}
Please take a look at this JSFiddle for a working example in which the label at Dec 2019 is shown in the navigator.
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Sachin Bisht
Team CanvasJS