The only workaround that I have been able to successfully use for this is to manually replace the #HOST# in the source of a chart region with &HOST. - which references an APEX Application Item that I set when the user authenticates. This does work reliably, but you do have to make sure that you update the HTML source of each and every chart and replace BOTH instances of #HOST#.
A nice enhancement request would be to make #HOST# an instance-level variable, as you will also run into this issue when using a reverse proxy. This issue also exists with charts in Interactive Reports, and I do not have an easy to implement workaround for that.