hairline(width) specifies the width of hairlines (lines with nominal width 0) as rendered to screen or pdf.
The default is 0.25 pt when rendering to pdf. When rendering to screen, the default is 0, which implies "one pixel wide".
Unlike all other QPlot commands, this one is effective retroactively as well as proactively: It affects every single object in the current figure that is plotted with nominal width 0.