Modify the legend. LABEL= suppresses the legend label. SHAPE= defines the size of the legend values. CBORDER= draws a black frame around the legend. CBLOCK= specifies a gray block that matches the 3D planes. ORIGIN= specifies the same position as in the AXIS1 statement. 
legend1 label=none
        shape=bar(3,3)
        cborder=black
        cblock=gray
        origin=(24,);