I just posted about this same problem earlier today, too.
The thing is, Perspective uses the AMCharts library for charting, and while AMCharts itself DOES support text rotation in bar chart labels, Ignition’s front-end to AMCharts does not let you configure this.
So the short answer is: No, this isn’t possible with the current charting.
However, I’ve heard that they’re revamping the charting and will possibly have options for this soon.