Don't issue time or hashtag queries when those charts are minimized

[tmostak]

As a user I should only pay a performance hit on charts I am actively looking at. In particular, the hashtag query is currently expensive as the Twitter dataset gets large (think > 1B rows) and so it would be nice for demo scenarios to be able to use a bigger dataset by collapsing the hashtag chart. Also on mobile the hashtag chart is not shown by default so its even a more common scenario of query wastage.