In reply to LeRoy Bessler.

Thanks for writing. The short answer for the popularity of gradient color ramps is that, in most cases, we don't need to "match the color" to a value nor to obtain a "precise estimate in the difference." Heat maps provide an effective way to visualize when cells are different or similar to each other. You can also obtain an approximate magnitude of the value. We are often interested in whether variables are "not very correlated", "moderately correlated", or "strongly correlated," which you can see without displaying numbers.

Yes, for more precise information, I agree that tables are better. As you point out, you can use the TEXT statement to overlay approximate values if the cells are large enough to contain the text. I've used this technique in several blogs, including one (a visualization of the "Wind Chill Chart") that varies the color of the text in order to make the text visible on light- and on dark-colored cells. This avoids using backlighting.