One of my main problems with Numbers is the fact that it has much less of the functionality of Excel. That's why I barely use Numbers.
One of the main things that I always thought impossible was plotting two axes on one graph. This is my main reason for my dislike of Numbers.
Now that many other interesting questions that I didn't think had answers are getting answered, I thought I might ask this question:
How can I plot two lines on one graph using a secondary axis in Numbers?