In update method, you are updating the last dataPoint’s y-value only and not x-value because of which x-value remains same and being shown in crosshair as such. Updating x and y-values should work fine in your case. To keep axis labels constant, please take a look at this sample dashboard. You can download the dashboards sample from our download page and try customizing and running locally.
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Vishwas R
Team CanvasJS