You can add geom_vline(xintercept = c(-0.75, 1.00, 2.75, 4.25, 6.00, 7.75), linewidth = 3, color = 'gray92') + below your firt geom_vline
geom_vline(xintercept = c(-0.75, 1.00, 2.75, 4.25, 6.00, 7.75), linewidth = 3, color = 'gray92') +
geom_vline