I dug couple of hours until I found a solution.
The problem was with the string itself. when I wrote on my html file "[legendPosition]="'below'", it just did not recognize it, i.e. the request's content.
on html page (on ngx-charts tag):
[legendPosition]="legendPosition"
in its corresponding ts file, inside the exported class:
public legendPosition: LegendPosition = LegendPosition.Below;
and of course, this won't work unless you import LegendPosition:
import { LegendPosition } from '@swimlane/ngx-charts';
Now, instead of receiving a string, [lgegentPosition] will use a LegendPosition object.
By the way, you can see that I have used "LegendPosition.Below". This is not the only way to present the legend, but just an example.