In case running Jenkins with reverse proxy, we can see graph images without any problem but the links to the job are not correct.
It seems the link is not based on Jenkins URL (at main configuration) but on the request header from Apache to Jenkins.
I wonder it's better to use not getRootUrlFromRequest() but getRootUrl() in DotStringGenerator#getEscapedProjectUrl.
I'm sorry this is because my setting of Jenkins.
(I referred to Jenkins Wiki - Running+Jenkins+behind+Apache - and set it up.)