The appearance of graphs can be customized using the third button from the top on the graph toolbar. Pressing this button will open a window that allows you to change just about any aspect of the graph: series, axis, title, legend, etc. When you close the window your settings will be saved. Whenever you use NEM-Watch, it will load the graph as you have configured it.
The fourth button from the top on the graph toolbar allows you to discard your custom settings and revert to default settings. Use this button to get yourself out of trouble if the graph starts acting strangely.