It really depends on what you want to do with the colours. If you're plotting multiple plits in a single graph, you need way different colours thanfor a good colour scheme you'd make a poster design with.
Since Zach already provided good colours for the contrasting-graph scenario, I'll add my advice for creating a balanced colour scheme. Since the current colour scheme already includes four different hues, I'd shy away from adding any more. Try and add multiple levels of white and black to your existing colours, creating tints (mixtures with white), tones (mixtures with black) and shades (mixtures with grey) of your existing hues. You essentially change the Saturation and Brightness/Value numbers for your existing colours, while keeping their Hue value intact.
Colours with the same hue value, but different saturation and brightness (or value) always go well together.