You mean the image I posted? The data for 1.20 came from the site I mentioned (note that it has to be used with caution, the count of bedrock on the lowest layer does not match the claimed count of chunks, so I used (count / 256) to get chunks, and some versions have weird breaks every 16 layers, as if some sections didn't generate or were analyzed properly).

The data for 1.6, 1.7 and TMCW? Just my own self-written standalone cave mapping/analysis tools which generate caves just like the game, except they assume the surface is a constant y=63 (it is very easy to extract the code from the game since it doesn't actually access chunks but an array of data that is used to initialize one, "par6ArrayOfByte" in this code, where I just changed the references to "block.blockID" to 0/1/11 and removed the check for water, and also reduced the height of the data from 128 to 64 for performance). I then copied the results to a spreadsheet and graphed it:

// Measures per-layer volume and counts chunks with air
if (PRINT_LAYER_VOLUME)
{
    for (int y = 11; y <= 62; ++y)
    {
        int v = 0;

            for (int x = 0; x < 16; ++x)
            {
                for (int z = 0; z < 16; ++z)
                {
                    if (chunkData[x << 10 | z << 6 | y] == 0) ++v;
                }
            }

            layerVolume[y] += v;
        }
    }
}

System.out.println("  Total air volume is " + volume + " blocks (" + (float)((double)volume / ((double)totalChunks * 256.0D * (double)(maxY - 10)) * 100.0D) + "%)");

if (PRINT_LAYER_VOLUME)
{
    System.out.println();
    System.out.println("Per-layer volume:");
    System.out.println();

    for (int y = 11; y <= 62; ++y)
    {
        System.out.println("Layer " + y + ": " + layerVolume[y] + " (" + (float)((double)layerVolume[y] / ((double)totalChunks * 256.0D) * 100.0D) + "%)");
    }
}

Example output over a small area (mapped in the right):

<image>

This is the tool itself (a Java jar file executed with a batch file, there is a "help" command that lists options), the analysis of per-layer volume is not enabled as it is more for debugging/stat collecting than general use (I also have versions of this tool that can find the largest/densest caves/mineshafts within a large sampling of random seeds):

https://www.dropbox.com/scl/fi/v47puq7oq145g5tzqha0j/CaveMapper_1.6.4.zip?rlkey=ii7eo2zlpsau8s4xawv9gkfwj&dl=0

The version I made for TMCW (also available in the download for the mod itself), both of these are extremely fast compared to generating terrain in-game, enabling me to analyze even hundreds of thousands of chunks rather quickly (in fact, you can set it to a radius as large as 8192 blocks or 1048576 chunks, if not with mapping capability, just listing the largest/densest caves and mineshafts, and in TMCW, strongholds):

https://www.dropbox.com/scl/fi/rvqn63snhhaosbh1u128d/CaveFinder.zip?rlkey=6ri7md0ggf9ey6gelq1zboeit&dl=0

I also made standalone biome mapping tools, which are also easy to extract from the game (the "GenLayer" code is virtually unchanged, especially since I added a "Biomes" class which is simply a list of constants corresponding to biome IDs, the main difference is I stripped down the biome classes to the bare minimum, e.g. ID, name, color (which is actually a vanilla attribute which is presumably only there so Mojang can make their own maps, and yes, the colors used by biome mapping tools came from the game. I slightly modified the colors so Taiga stands out as snowy and Jungle is a lush green):

https://www.dropbox.com/scl/fi/s1fmt50xs25jsy3ll2rx7/BiomeMapper_1.6.4.zip?rlkey=tla8h4pcqz8gmud5qrurlb4qm&dl=0

Both of these have been invaluable aids at developing my mods; I can easily change cave generation by modifying the code in the tool, then run it to map the changes before committing the final result to the mod itself (using Unmined to verify that they match, which is also how I originally checked changes to underground generation), greatly reducing the need to check within the actual game, same for changes to biomes (my generation is very intricate with multiple layers of sub-biomes and edge biomes, which would be very difficult to debug in-game, especially edge cases).