How to set the length of two ticks in DateTimeAxis ?

You can use the `Transform` method of the axis to get the length on the screen, but it will be a bit difficult to calculate the scale of the other axis. I think you need to consider the width of the control, the margins, and set the `Minimum` and `Maximum` of the second axis.

I think this should be covered by the following feature request: https://github.com/oxyplot/oxyplot/issues/13