In a cell A1 the text can use different styles for different parts of text.
For example, "abcd" can have 'a' as bold, 'b' as underlined, 'c' as italics,
and 'd' as normal. This is rendered correctly.

If a chart has a reference to A1, e.g. in a Label1 of X-Axis1, the text is 
shown just as text, and only whole text could be bold, italic, or bold-italic 
(but not underlined). 

I believe this is a bug, and the same will happen when upper/subscripts and 
other text styling are implemented, if not fixed.