Hi @Jemster ,
Not sure if I exactly understand what you're after, but I made a mock-up table with 9 different tools (instrument) and 3 different plants and a data column which represents the values for each tool. I've also added arbitrary spec limits on the Data column.
Using Graph Builder, make a heatmap of tool vs plant and using the Data (value) column as the Color option of the heatmap get the basic structure of what you're after. If you then right click the color legend and select Gradient. You can then define the minimum and maximum values for the gradient, as well as define a set number of levels if desired (discrete rather than continuous). I used the spec limits as the min and max of the legend. Then, if you click on the color bar by Color Theme and choose a divergent color scheme, then the colors that are closer to the target are a neutral color while the ones further away diverge to red/green (in my example). See the screenshot below.
This might be a way for you to do it. I'm sure there are other ways, but this is what came to my mind. Within Graph Builder, there are a lot of modification you can make manually to change the way a graph looks. I've added the table and saved the GB script to the table if you want to check it out.
Hope this helps.
Good luck!,
DS