Examples of line-flux measurements for nebular lines at different signal-to-noise level. In each panel, the background is the two-dimensional spectrum, where the spatial direction is plotted against the right y-axis. The color coding is indicated by the color bar. The one-dimensional spectrum with a Gaussian fit are shown in black points and line. Gray data points are excluded from the fit because of telluric or skyline contamination. A telluric transmission spectrum is plotted in a thin black line at the very top of each panel. From top left to bottom right: [O ii]  (GRB 140506A, with afterglow background), [Ne iii]  (GRB 100424A), [O iii]  (GRB 070129), Hβ  (GRB 051022), Hα  (GRB 070110), [N ii] (GRB 050416A).