Changed the arguments on drawSeries. Series now hold their index in the
graph. _values is now protected. Some small optimizations were
implemented. Many more are needed. getMax/Min are very inefficient, and
they should have values passed to them, rather than recalculating value
list. Value list is also dependent on graph viewport size, yet horizontal
lables don't seem to care. I haven't actually played with the graph
zooming, but it seems like the solution to drawing scaled graphs is
convoluted and confusing.