Analysis of the relationship between the feature "Number of grains per plant (NGL)" and the task of predicting the grain yield of a soybean cultivar:

Based on prior knowledge, the number of grains per plant can have a significant impact on grain yield. Generally, a higher number of grains per plant would indicate a higher grain yield, while a lower number of grains per plant would indicate a lower grain yield. However, it is important to note that other factors such as environmental conditions, nutrient availability, and disease incidence can also influence grain yield.

To complete the dictionary, we need to include 5 typical NGL values for each target class ('low', 'high'). Since there is no specific information regarding the value ranges for NGL, we will consider the following assumptions:

- "low" grain yield: NGL values ranging from 50 to 200
- "high" grain yield: NGL values ranging from 400 to 800

Based on these assumptions, the dictionary can be generated as follows:

```json
{
	"low": [50.0, 70.0, 100.0, 150.0, 200.0],
	"high": [400.0, 500.0, 600.0, 700.0, 800.0]
}
```