Vertical rotational gradient in the inner Milky Way. Thin lines: rotation velocity v_φ(z) averaged in the region 3.5 < R < 5.5   kpc derived with the LAB dataset using the tangent point method. Each curve stands for a different region. Error bars are not shown. Black points: average v_φ(z) curve. Thick red line: linear fit to the average curve. The yellow region stands for the 2σ error on the gradient (1σ = 4   km   s^-1      kpc^-1).