Clip rect views

This is a fix for PT bug, https://www.pivotaltracker.com/s/projects/38251/stories/50688287 , "Data shows beyond the top or bottom of the graph pane".

I've added clipping rectangles that match the max/min values of the X and Y axis to two views, connected_line.js and point.js. 

This fixed this specific bug but we may want to add this clipping rectangle to other views so that we don't draw above and below both axis.