...or do I need to raise an enhancement request?
Allowing only horizontal text labels only on x-axis point labels means that more often than not for the kinds of things I am charting, labels are compressed to a useless "start...end" name.
A common example is when creating distribution graphs with buckets named like "0-100", "1000-2000" etc. When you have more than a few of these, the number (and length) of the labels required means that the labels are unreadably truncated to "0...00" or "10...0", and you have to mouse over a data point on the chart to see what the value is.
Having a vertical label option would solve all of these problems as you then have a very small fixed width for all x-axis labels (ie. the height of one character).