How can i group calendar subscriptions in Mavericks?

In Lion, I was able to put all of my calendar subscriptions into groups. This was quite useful to me for schedule planning.

Since switching to Mavericks, I'm unable to put subscriptions into groups. All of my subscriptions appear in the sidebar under "Other", and even though the Location of my subscribed calendars is "On My Mac", I'm unable to put them into groups that are "On My Mac".

Can anyone tell me how to get my calendars back into groups?