allow legend configuration for each series

Introduces showInLegend field in GraphViewSeriesStyle. This can be
configured, and will be used to determine if a series is shown in the
legend, or not. In this way, multiple series can be overlaid to create
complex graphs, while retaining a single label in the legend.
allow value dependent color for line graphs

Updates LineGraphView drawSeries() method to request
getValueDependentColor() from each series when plotting the graphs.