Heat gradient doesn't show heat gradient! It shows contour lines that are more compressed in areas of higher heat gradient but it doesn't show heat gradient.
To show magnitude of heat gradient, You find the temperature at one point and the temperature at an adjacent pixel in the horizontal direction and subtract the 2 and take the absolute value. Then you do the same thing but for an adjacent pixel in the verical direction. Then you square both of these values, and then add these 2 new values together, and take that value and perform a squareroot on it. The resulting value now is the heat gradient magnitude. A true "heat gradient" display would in turn take this value and use it to assign a brightness value to the pixel ranging from black (DeltaT=0) to white (DeltaT=sqrt(9999^2+9999^2)). Now THAT is what the correct "heat gradient display" should show.

Alternatively you can show actual heat gradient vector field with color field like this:
DeltaTx=0 DeltaTy=0 color: red=128 green=128 total color: middle yellow
DeltaTx=0 DeltaTy=9999 color: red=128 green=255 total color: yellowish green
DeltaTx=0 DeltaTy=-9999 color: red=128 green=0 total color: middle red
DeltaTx=9999 DeltaTy=0 color: red=255 green=128 total color: orange
DeltaTx=9999 DeltaTy=9999 color: red=255 green=255 total color: bright yellow
DeltaTx=9999 DeltaTy=-9999 color: red=255 green=0 total color: bright red
DeltaTx=-9999 DeltaTy=0 color: red=0 green=128 total color: middle green
DeltaTx=-9999 DeltaTy=9999 color: red=0 green=255 total color: bright green
DeltaTx=-9999 DeltaTy=-9999 color: red=0 green=0 total color: black

And of course every in between set of heat gradient would be some in between combination of red and green values.

vanquish349:

How about we change the name to heat contour then.

That sounds better.