Sometimes multiple values are represented by single pixel co-ordinate. This happens when the number of values on axis is greater than actual pixels available in given chart container. For example: If the height of the chart is 200px and the axis range is 0 to 1000. Each pixel co-ordinate will represent 5 axis value co-ordinates.
You can overcome this issue by rounding off the selected value as per your requirement. You can also take a look at this example which uses ySnapDistance to round off the pixel positions.
You can also re-adjust the height and width of the chart and see if it makes any changes.