extra key entries

Normally each plot autogenerates a single line entry in the key. If you need more control over what appears in the key you can use the keyentry keyword in the plot or splot command to insert extra lines. Instead of providing a filename or function to plot, use keyentry as a placeholder followed by plot style information (used to generate a key symbol) and a title. All the usual options for title font, text color, at coordinates, and enhanced text markup apply. Example:

     set key outside right center title "Outcomes"
     plot $HEATMAP matrix with image notitle, \
          keyentry with boxes fc palette cb 0 title "no effect", \
          keyentry with boxes fc palette cb 1 title "threshold", \
          keyentry with boxes fc palette cb 3 title "typical range", \
          keyentry with labels nopoint title "as reported in [12]", \
          keyentry with boxes fc palette cb 5 title "strong effect"