The documentation under chart element attributed > Functional Attributes states: "For example, you can opt to show/hide data labels, data values, y-axis values."
The documentations also states that YAXISVALUES has a _alpha property, however showYAxisValues is not exposed for the dragline chart. It is exposed for the regular 2D line chart (and most other charts I have used).
Was this intentional? I realize that by default a user can change the Axis, so perhaps you did not want to allow the chart to hide the yVals, but there should be a way to hide those values, particularly if you set allowAxisChange='0'.
Thank you for your attention in this matter.
Best,
Anthony