BBC Poll Tracker is out of action, I wonder why

The BBC Poll Tracker is currently out of action, giving this message:

'Poll tracker

We're making some improvements to the poll tracker. While this is going on it won't be updated, but you can still look back at polls and key events over the last 30 years.

Alternatively, click here for details on latest poll trends.'

In case you're interested here's an up to date chart courtesy of Mike Smithson

Now why would the BBC not like to show that chart?