Based on my prior knowledge and understanding, I can analyze the relationship between the feature "Number of legumes per plant (NGP)" and the task of determining the grain yield of a soybean cultivar as follows:

Analysis:

1. Generally, a higher number of legumes per plant (NGP) is associated with a higher grain yield of soybean cultivars. This is because each legume represents a potential seed, and a larger number of seeds typically leads to a higher yield.

2. However, there might be an upper limit beyond which further increase in NGP may not significantly contribute to a higher grain yield. This is because the availability of resources and the plant's capacity to support a large number of seeds may be limited.

Based on this analysis, I will create a dictionary containing five typical NGP values for each target class: 'low' and 'high'.

```json
{
	"low": [2.5, 2.8, 3.2, 3.6, 3.9],
	"high": [4.8, 5.2, 5.6, 6.1, 6.4]
}
```

Please note that the above NGP values are for illustration purposes only and may not reflect the actual ranges specific to the soybean cultivar or the target classes. It is important to consult relevant research or domain experts to determine the appropriate ranges for NGP values in order to make accurate predictions about grain yield.