In a group component, the lines will only work with the Bar and Column display options.
Currently, like the lines option in the Time Component, they only draw a line at a specific value on the graph. They are meant to represent a goal or threshold. So, for example, if graphing the % uptime of services or % of tickets that met their SLA, your scale would be, say, 0-100 and want to draw a line at 95 (%).
Time Components do have another "line option" to display a trend line, indicating a data set's trending over the time span represented. Since the Group Component is not spanning time, trend lines are not as appropriate.
Here is an example of a Group Component showing each group's average incident time to close value with 2 lines representing different thresholds we've identified.