In order to hide past heat, Wikipedia pulls a couple of dirty tricks. They report the most recent year a record temperature was reached, instead of the first year. This skews the graph to favor recent UHI affected temperatures, and hides past heat.
But the real dirty trick is using bogus temperatures. The official temperature at Pierre, South Dakota on July 15, 2006 was 116 degrees – four degrees cooler than the Wikipedia claimed temperature, and four degrees cooler than the actual South Dakota record heat of 120 degrees on July 5, 1936. There weren’t any USHCN stations in South Dakota close to 120 degrees on July 15, 2006, but there were two on July 5, 1936.