How to display a percentage for a pie chart label using the Charts module?
This is a typical request for the Charts module ... There is currently no out-of-the-box solution available in the Charts module. But since you're using Highcharts, there is a patch available that may help somehow. The patch is attached to Comment nr 4 of issue #2209467. Be aware of the current status of this issue, i.e.:


the patch only works for Highcharts (as charting engine), the Google Charts counterpart of it is still missing (and prereq for adding this feature to the Charts module).
it is related to the Charts API (which is different from the Views integration). So after the Charts API feature gets delivered, some more (minor?) work is needed to also make it available within the Views integration of the Charts module.


The child issues of this issue contain some more variations, which may also help.

So only considering the Charts module as an alternative, and applying the suggested patch is not sufficient. But at least it should help somehow to understand what the missing pieces are. And maybe inspire somebody to submit 1 (or more) additional patch(es) to make it complete?

Note: The issue about "Views field as data label for better tooltips" is somehow related to this issue, and specific to the views integration. Comment # 6 in it contains an interesting patch for it also.

Disclosure: I'm a maintainer of the Charts module, I hope this does not violate the site's policy on self-promotion.