[Kuba](http://mathematica.stackexchange.com/users/5478/kuba) has already given a good answer, I just want to mention, if speed is really concerned:

    Graphics[{ColorData[1, "ColorList"][[1]], Point@Flatten[a, 1]}, Axes -> True, 
      AspectRatio -> 1/GoldenRatio] // AbsoluteTiming

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/cMSNP.png