Passing axis interval as pixel is not possible as of now. However, you can calculate the interval in pixel based on axis bounds and axis range (viewportMinimum and viewportMaximum) as shown in the code snippet below.
chart.axisX[0].set("interval", (chart.axisX[0].viewportMaximum-chart.axisX[0].viewportMinimum)/(chart.axisX[0].bounds.x2-chart.axisX[0].bounds.x1) * intervalInPixel)
Please take a look at this JSFiddle for an example on the same.
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Manoj Mohan
Team CanvasJS