Your image, though, does have some issues, as there are some locations that are easy to get to, but no way to get out of (99% x, 20% y)
I like that you were able to spot issues in the image.
Edit: Better arrows:
I like this graphical representation of things! It brings back (painful) memories of Master of Orion 3 and their graphs for planet habitability (T_T That game had so much potential... if only there was another Toady who worked on that game... but, I digress).
But, yeah! I had an idea this morning, and I believe something similar may have been suggested before on this thread but I can't find it...
So, I reaaaally like the idea of getting a loose simulation of soil chemistry and the effects of fertilizers, moisture, biomes, etc. into the mix here. I can assume that, if implemented, a lot of the stuff might happen in the background (as in, there isn't an indication of exact pH, potassium levels, nitrates, etc. directly observable by entities in the game or the even the player... just loose indications sort of). The graph you presented shows an interesting way of perhaps graphically representing what crops are good where and what the effects might be of planting this that or the other crop or using fertilizers before or after, and I could roll with something like that. But I don't know if it would fit in the scope of this game in terms of it being medieval-esque; the axes of the graph might represent stuff beyond the immediate knowledge of medieval farmers.
BUUUUUT!!! It gives me an idea of tying in two things that earlier on NW_Kohaku and I (and I think others) thought would sort of be impossible to do. Those two things are:
1. Farmer skill and technological know-how (i.e. knowledge of different farming methods, tools... maybe something implementable later... but that's in another thread)
2. Player design of farms, including irrigation systems...
So, the player presumably will know the merits of crop rotation, having grown up in the 20th and 21st centuries, as well as the merits of irrigation. Crop rotation as it is today is a technology developed past the 1400's tech time frame of this game (though forms of crop rotation and irrigation were performed during medieval times and throughout history for that matter). Crop rotation today involves knowledge of the chemistry mentioned earlier in this thread, which is not something known by medieval peoples. But, farming peoples back in the day would know, either through personal experience or the experience of their peers and forefathers, more or less what crops would be safe to plant at what time and where, and what crops might be best to plant after another crop, and when to leave a field fallow. They would also know when a plant looked healthy vs. not healthy. It wasn't as perfect of knowledge as what we have now, but it was good enough.
I think we could treat the system of what is displayed to the player about plant health (whether its getting enough water or sun), and soil and weather quality (this plant would grow well here and not well there) much like how quantities of stockpiles and fortress wealth are handled; the skills and knowledge(if that's ever implemented) of the farmers would determine the relative accuracy of the information presented to you (maybe even in the form of the graph shown by AngleWyrm!!!) and what your farming dwarves might suggest to you to increase crop yields (needs more of X fertilizer, needs more(or less) water, this crop might be better planted after this one). The factors that govern the needs of the plants in the background are based on a model of RL soil chemistry, while the knowledge of the dwarves and what info is presented to you would be based on what info would have been available to farmers during medieval times.
Hooray! What ya think???