Mirror images

On Twitter, Hans-Jörg Schulz put up a nice example of two charts. The first chart, shown above, paints a picture of an influx of immigrants into the United States, since the 1970s. We also notice that the proportion of immigrants was similar to the present share if we look back to the 1920s. The trend is U-shaped, first falling, then rising, now reaching a level comparable to a century ago. Depending on one's political views, the message may be alarming or optimistic. Here is the second chart:This chart tells a different story. The trend is relatively flat. The proportion of non-immigrants rose a bit then fell back to the level in the 1920s. The reader may think this is a story of stasis. Of course, the underlying dataset of both charts is identical. Each person is classified as either an immigrant or not an immigrant. What accounts for the different messages?The first difference is what's on the chart vs. what's not. The first chart shows the immigrants while the second shows the…