You have to give <b> for bold, <i> for Italic & <u> for underline.
From Qlikview help file:
Edit the Text Format expression to enter an attribute expression for calculating the font style of text associated with the data point (For tables: text in the table cell for each dimension cell. The calculated text format will have precedence over table style defined in the Chart Properties: Style.) The expression used as text format expression should return a string containing a '<B>' for bold text, '<I>' for italic text and/or '<U>' for underlined text. Note that = is necessary before the string.
Hop this helps you.
I wasn't aware that in previous versions there was a function bold().
Just discovered it in charts that were designed by previous developers in my company.
Apparently, this function was supported up to ver.11 SR10.
Can't understand why Qlik replaced it with such unfriendly attributes.