Support for redrawing chart from the external script.

Hello :c) 

thanks for this great library. It had just one inconvenience - it wasn't possible to redraw chart from the outside - for example when you changed size of the browser. It could be possible with this little patch. 

I "extracted" function for drawing, so it's not anonymous anymore, so you can call it from your own script. Name of the function is defined from the element_id, like this:

draw_#{element_id}_Chart()

If element_id contains dashes, they are converted to the underscores.

Have a nice day :)