Add a Close button on the stacked bar chart panel

[philrz]

At the time this issue is being opened, Zui is at commit 205c88c.

When using Zui for the first time in a while, @mccanne asked the following question:

Anyone know how to make the barchart area go away after clicking on the button to create it? Even if I close the tabs, quit, and restart zui, the erroneous div is still there...
Unable to determine date range using 'ts'.

He soon recognized this button could be clicked to toggle it off.

In thinking about why he hadn't tried this initially:

I clicked it once and didn't realize it was a toggle and looked for a close button on the panel. Maybe have both?

Upon hearing this, @jameskerr concurs.

Agree, there should also be a close button on the panel.