Based on prior knowledge, the number of grains per plant (NGL) can be a good indicator of grain yield in soybean cultivars. Higher values of NGL would generally indicate a higher grain yield, while lower values of NGL would indicate a lower grain yield.

To analyze the relationship between NGL and the target variable (grain yield), we need to consider the ranges of values for NGL that are typical for low and high grain yield.

In this task, we'll assume the following:
- Low grain yield corresponds to NGL values below 100.
- High grain yield corresponds to NGL values above or equal to 100.

Based on this analysis, we can create the dictionary:

```json
{
	"low": [10.5, 20.2, 30.8, 40.9, 51.1],
	"high": [108.3, 115.7, 120.2, 133.6, 150.9]
}
```

These lists include 5 typical NGL values for each target class ('low' and 'high'). Note that the values provided are just examples and may not be specific to the soybean cultivar being analyzed. The specific values will depend on the dataset or domain knowledge.