There is a way to increase the resolution when the plot is exported with
export_png, that is setting width and height directly in the method:
export_png(p2, filename="plot.png", height=300, width=300)
The problem is that the axis keep always the same size, no matter how big the plot is:
export_png(p2, filename="plot.png", height=1200, width=1200)
The possible solution right now is to update the axis attributes manually. I think an easy fix would be to increase, in proportion, the size of the axis and title elements when exporting.