The title acts as a separate box placed above the chart. The white line is actually the colour of the dashboard background behind it. 2 ways to rectify this would be:
- Choose Dashboard from the menu bar, click 'format' and then shade your dashboard to the same colour as your chart background.
- Right click on your chart, click 'format', click the little paint can and then shade the graph to the colour of your dashboard.
Please let me know if this solves the issue.
Try right-clicking on the title, then format the title. You should be able to change the color from there.