This tool was originally located here as a somewhat hidden gem on Steembottracker.com. As you can see, that version is broken and hasn't worked for quite some time. I dug around a bit and found the code for it available here on Github. The application was originally written by MattyIce (@yabapmatt).
I wanted to put this up because it's the only tool out there that I've come across which allows you to visualize, in real-time as your HP earns interest. I've added @yabapmatt as a beneficiary on this post. Hopefully he has no objections to my re-purposing of this application!
Your Hive Power is not technically gaining interest. Unlike liquid Hive, Hive Power (vests) is protected from inflation.
As the total number of Hive in existence increases, your Hive Power will increase so that you still own the same percentage of the total coins. So, for example if you own 1% of all Hive in existence as HP, your Hive Power will increase with each newly created block to ensure that you always continue to own 1% of the total coins.