...
Once entered, a message will appear saying that no entity was found, press the 'Create a new one!' hyperlink.
From the window that appears chose 'Single entity', 'Device' and then finally select the 'E39_A_Sulafjorden_wave' device from the drop down menu. Finnish by clicking the 'ADD' button.
Clicking the 'Timeseries' section will now prompt a list to appear. The list contains variables collected from the Sulafjorden buoy. Looking at the data section we can learn more about what the variables represent.
Select 'Hm0' which is described as "Significant wave height estimate from spectrum".
It should now look like this.
To add more variables from the same entity, simply click the 'Timeseries' again. Now select 'thmax', which is described as "Period of highest individual wave from zero crossing analysis".
Once completed, hit the 'ADD' button at the bottom of the window. This should create a chart widget at the dashboard with two variables plotted.
To finalize the dashboard click 
'Apply Changes' at the bottom right.
You can add multiple entities and compare data from different sources in the same widget. It is also possible to change the color and look of the widget. This can be done under the 'Settings' tab for the widget.