Quick hack for addressing issue #30.

svg.py: Added the viewBox height by 20 so that I can put X-Axis title
there.
config.py: Added xtitle and ytitle for giving title name for the
respective axes.
graph.py: Added functions _x_title() and _y_title() which adds title
for the respective axes.
Fix x/y title margin computation + split if too long + position with legends