The map shows the years in which the highest average July temperatures occurred in the US during the last 34 years. (*)
Record heat can mean many things. For example, it might mean a record temperature reading at a specific location and at a specific time. However the impact is often greater when heat is persistent and felt over a wide region. The map is based on spatial (state) and temporal (month) averaging of temperature data. Of course by any measure, record temperatures have been experienced in the US corn belt this summer.
In terms of impact on agriculture, the heat wave/drought of 2012 is comparable to the severe drought of 1988. Surprisingly, 1988 does not show up as a record July temperature year. 1980 is still a record year for much of the Southern Plains. The Florida heat wave of 1998 (an El Nino year) also remains a record.
(*) The map is based on 100km reanalysis model data which means that surface temperatures are inferred from satellite radiances. To produce the map, I took a spatial average for each US state.