As it was mentioned before, graphs are plotted automatically for all numeric values from Data table.

You may wish to add graphs in the following cases:

These cases can be combined.

So, to add new graph click  button over the list of graphs. You will see the following panel with the current object and its numeric metrics:

Then choose the metric from the current or any other object. To do this, click on the desired metric or in the "Formula" field start entering its name, written in the figure braces, and select the metric in the filtered list. Then click  button: 

To add the multigraph just choose several metrics from the current or any other object using  and  buttons:

Multigraphs can be used to plot additional graphs for constant valus, e.g. to display max and min levels for the metric: