Here is a dark and moody color palette suitable for line graphs:

 <Dark and Moody>
1. Midnight Blue - 
                            
                            #001F3F
                         - A deep and rich blue that evokes a sense of calm and depth.
2. Charcoal Gray - 
                            
                            #333333
                         - A dark gray that provides a neutral backdrop, enhancing readability.
3. Burgundy - 
                            
                            #7B1F3A
                         - A dark red that adds warmth and intensity to the palette.
4. Forest Green - 
                            
                            #2E4A2C
                         - A deep green that brings an earthy and natural feel.
5. Steel Blue - 
                            
                            #4682B4
                         - A muted blue that offers a cool contrast to the darker colors.

This palette creates a sophisticated and dramatic atmosphere, perfect for line graphs that need to convey seriousness and depth. The colors are distinct enough to differentiate between multiple lines while maintaining a cohesive and moody aesthetic.

Would you like to modify any of these colors or need additional options? What types of data will you be displaying in the graphs?