What color would you like the cells with no values to be, I'm guessing white? If so you could add gridlines to the axis and add the heatmap line color, then you should see every thing filled in:
Here is a script to recreate these values:
Names default to here(1);
dt = Open( "$Sample_data/big class.jmp" );
dt << Graph Builder(
Size( 550, 482 ),
Show Control Panel( 0 ),
Variables( X( :height ), Y( :weight ) ),
Elements( Heatmap( X, Y, Legend( 4 ) ) ),
SendToReport(
Dispatch(
{},
"height",
ScaleBox,
{Label Row(
{Major Grid Line Color( 0 ), Minor Grid Line Color( 0 ),
Show Major Grid( 1 ), Show Minor Grid( 1 )}
)}
),
Dispatch(
{},
"weight",
ScaleBox,
{Label Row(
{Major Grid Line Color( 0 ), Minor Grid Line Color( 0 ),
Show Major Grid( 1 ), Show Minor Grid( 1 )}
)}
),
Dispatch(
{},
"Graph Builder",
FrameBox,
{DispatchSeg( RectSeg( 1 ), Line Width( 1 ) )}
)
)
)