[IMP] (Legend Tooltip) only show legend tooltip when the text of the legend item was trimmed.
[FIX] (Pie Panel Legend) When all the categories were set to invisible, by using the legend, the panel would internally throw and invalid data exception, which resuted in a chart with no legend to be able to make categories visible again.
Changed the level at which the exception is caught, so that it shows on each plot's panel instead. Also, now only throwing the exception when there are any categories, but their values do not allow representation.