For some reason the reply above seems a bit screwed. What I meant to say was that I'm very aware of when and how Stockholm was founded, I used the name Stockholm because I didn't think you'd know the actual name of the island on which the old town is located; Stadsholmen.
hte point is that next to none of islands in Mälaren or Roslagen are big enough to by themselves be visble on that map, even more so keeping in mind that they
were significantly smaller back then.