@afreay,
Axis labels are shown at every interval which is auto-calculated based on parameters like axis minimum, axis maximum, etc. In your case, since the range of Y-Axis is small the labels are shown in decimals. Hence, using the valueFormatString will just round off the labels to the nearest whole number but won’t affect the interval, as a result, the labels might get repeated.
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Indranil Deo
Team CanvasJS