Make random slopes line bigger

[rikfor]

Hi Dustin, and thank you for a great package!
I'm generally new to R and learning about how to make various nice LMM-plots with flexplot. One thing I can't figure out is how to make the random slope lines bigger, for example when plotting:
visualize(model, plot = "model", formula = MathAch~SES + School))
(full formula is MathAch~SES + Minority + (SES|School))

As I understand it, the black line is the "mean effect" for SES, and the colored lines is the random slopes for SES. But for me it is hard to see the colored lines as they are much smaller than the black line. Any way I can improve this?

Thank you in advance!