vs. L_opt for all groups in our final sample, with 1σ uncertainties shown. The trend line from Fig. 4 is shown as a solid black line. The dashed lines show the thresholds used in creating our X-ray overluminous and X-ray underluminous samples. These lines are a factor of 2 above and 3 below the best fit line, respectively. These values were chosen to achieve comparable numbers of groups in the bright and dim samples. More discussion on different choices for this selection can be found in Sect. 5.