rv diagram of supershells. Evolutionary lines (Weaver et al. 1977) for three luminosities are overlaid (dotted lines), as well as three lines of different times (dashed lines), for the density n₀ = 0.3  cm^-3. Different symbols denote estimated energy inputs to shells, when densities from Kalberla & Dedes (2008) are taken into account (see text for the explanation): crosses for L < 0.1  SN      Myr^-1, open circles for 0.1  SN      Myr^-1 ≤ L ≤ 1.0  SN      Myr^-1, triangles for L > 1.0  SN      Myr^-1.