[o][h]plot_with_err improved (set_ranges, histogram, error bars)

- set_{x,y}range / set_ranges qualifier can now be used to specify
  a fraction of the max-min range that is padded onto both sides of the range;
  this fraction defaults to 0.05

- histogram lines are now drawn correctly
  even if data points are outside of the plot window.

- error bars are now drawn correctly
  even if corresponding data points are outside of the plot window