This is indeed very tricky
Step1: Control+Shift undock the Legend from the side and bring it in an area where you can easily adjust it
Now if you directly dock it to top or bottom you will not see all legend because, look at screenshot 1, we have not adjusted width of the legend. So always extend the legend to the full width of the chart.
Now we stretch the legends when it is not docked as below
Now we dock it to the bottom again and Voila!
or at top