Lines - whether they are drawn connecting points of a curve, the bins of a histogram, or the axes of a plot - have a thickness attribute, which controls how thick the lines are drawn. The attribute can take a value between 0.5 and 10, although the difference between 0.5 and 1 is only discernable in the vector output formats (i.e. postscript and PDF).

To make the line of a curve thicker than its default value (which is 1) you would say:

# Python
  set_curve(["line.thickness",2])

% S-Lang
  set_curve({"line.thickness",2});

To change the axes of the current plot to use a thickness of 2 you would say:

# Python
  set_cascading_property("all",chips_axis,"thickness","2")

% S-Lang
  set_cascading_property("all",chips_axis,"thickness","2");

The use of set_cascading_property() rather than set_axis() here is because there are multiple parts of an axis with a thickenss attribute, such as majortick.thickness and majorgrid.thickness. In this call the value has to be given as a string, hence the quotes around the value.