KubaKuba has already given a good answer, I just want to mention, if generating speed is concerned:
Graphics[{ColorData[1, "ColorList"][[1]], Point@Flatten[a, 1]}, Axes -> True,
AspectRatio -> 1/GoldenRatio] // AbsoluteTiming
But sadly the displaying speed of the above result seems to be slower than that of ListPlot
. See the comments below for more information.