Re-use already created jqplot object when re-plotting, and added a destroy process.

[yoichiro]

The original implementation does the removing all child elements when re-plotting. I guess that the way is heavy. Instead its way, we can use jqplot.replot() function for re-plotting.

Also, the original implementation does not call "destroy()" function of the jqplot object. If the element was removed, then the jqplot should be destroyed before the element is deleted.

Would be great if you merge my pull request. Thank you for your reviewing.