For a pie chart im using <cewolf:legend> tag to generate legend. I have written post processor for the original pie chart. and i have modified the look and feel of the legend. It looks fine if i give showlegend="true". But if i render the legend alone by giving the chartid attribute of <chart:img> tag to the 'id' attribute of <cewolf:legend> tag..
I've been looking at the same problem. I've been reducing the font size in the legend as I find the default a little large for my charts - this works fine when the legend is attached to the chart, but when separated the font size stays as the default.
I'm guessing this is because the post processor is linked to the chart, but the legend just reuses the dataset and hence doesn't have a link to the post processor.
According to the cewolf tag documentation the <cewolf:chartpostprocessor> is not supported under the <cewolf:legend> tag (It doesn't state this explicitly but doesn't state that it can be done), and in fact if you put there it appears to be ignored (i.e. it doesn't fail, but doesn't change anything either).
I'll continue looking for a way and will post anything I find here. If you find a way of doing this (or if anyone else does ;-) could you post it here too?