Currently there is no mechanism to draw the axis labels vertically, or to control the angle at all. This is an issue that has come up before, and we are looking for ways to solve it. One problem with using large vertical tick marks is that they bleed into the x-axis label and have a larger chance of getting cut off.

I would suggest if possible, only plotting every other tick mark, which you could achieve by using the tickFormat parameter on the x-axis scale. You would have to figure out all of the ticks you want to display first and store them in an object so you can check to see if it's a tick that you want to display or not.

Hopefully that can help you. If not let me know and I'll see if there's anything else we can do.

Thanks,
Alan