Based on prior knowledge, the Number of grains per plant (NGL) is likely to have a positive correlation with the grain yield of soybean cultivars. A higher number of grains per plant would typically result in a higher grain yield.

To analyze the relationship in more detail and identify typical NGL values for each target class, we would need access to relevant data. Without specific data, we can provide a general approach based on common observations.

Here is a dictionary that includes 5 typical NGL values for each target class:

```json
{
	"low": [20.5, 22.8, 23.9, 18.3, 19.6], 
	"high": [35.2, 32.9, 38.1, 34.6, 37.8]
}
```

Please note that these values are merely examples and should not be considered as accurate predictions. Actual NGL values for each target class can vary depending on various factors such as soil conditions, climate, genetics, and cultivation practices. In order to provide a more accurate analysis, specific data and analysis would be required.