Make sure user selects at least two times before going ahead with creating Time Series Chart

Hey guys - sounds like there is a release coming soon?  If too close to code freeze and you want to wait on this I understand, but it's pretty benign.  Covers this McV Inquiry:

http://dcdbs.ssec.wisc.edu/inquiry-v/index.php?inquiry=1246

I agree with Bob, no sense plotting an empty chart.  Here's a screenshot of new behavior:

![screen shot 2013-07-31 at 2 38 41 pm](https://f.cloud.github.com/assets/2334847/889174/4c2fed66-fa1a-11e2-8446-c08407051f04.png)