I'm not sure I quite understand the issue you're facing.
However from doing a little bit of searching, it appears the recommended way to get an italic version of the system font is to use font descriptors:
let xAxis = SChartNumberAxis()
let fontDescriptor = UILabel().font.fontDescriptor().fontDescriptorWithSymbolicTraits([.TraitItalic])
xAxis.style.titleStyle.font = UIFont(descriptor: fontDescriptor, size: 30)
xAxis.title = "X Value"
chart.xAxis = xAxis
This produces the expected result - the x axis label is a larger than usual size with the font italicised.
Additionally, doing something like UIFont(name: "Baskerville", size: 40) also meant the axis title was rendered with the expected font.
If I've misunderstood your issue, please let me know and I'll take another look.