Small multiples can work wonders when data are replicated, as in this case. The chart accompanied an Economist article on pollution levels in several European cities, as indicated by the concentration of nitrogen dioxide and particulates.
In the junkart version, I plotted the data series side by side, rather than one over the other. Further, the order of cities was according to decreasing levels of NO2, which seemed to be the worse pollutant. All gridlines are removed except the 30 line which worked pretty well to separate out the highly polluted cities.
An odd pattern has now surfaced. Namely, there is some degree of negative correlation between the concentration of the two pollutants. Environmental scientists may be able to tell us why.
Reference: "The Big Smoke", Economist, Feb 3 2007.