Revert "canvas zIndex needs to be 2 so realtime graphs show the data lines"

This reverts commit c9c7c678ad126c181b685cb0012c853e0859a041.

The canvas has to be under the SVG plot rect so the plot rect
can catch pointer-events in IE9 and IE10.

The plot rect needs to be transparent to be able to see the
plotted lines in the Canvas element.