如何在d3中指定轴刻度标记的数量？

I have created a simple bar chart in d3 and format the y-axis with '.ticks(3)', indicating I'd like to see 3 tick marks. However, depending on the data, I sometimes see 3 tick marks and sometimes see 4. Also, even when I do get 3 tick marks, the ticks are at around 1/4 up the axis label, then another at 50%, then another at 3/4. How do I force d3 to set 3 axis labels, one at the bottom, one at the middle, and one at the top?I have created a simple bar chart in d3 and for