y-axis changes

add 'regular' option to start y-values at 0 instead of min y-val
change height defaults
  now graphs showing a range of y-values smaller than 20 are
  shown with a shorter height
related fix: modify drange calculation slightly to prevent undesired
  side effects of float arithmetic
modify x-label printouts

trim x-labels to smallest non-zero decimal, and drop decimal place if
  the x-value is an integer
Merge pull request #14 from mc818/xlabels

modify x-label printouts
Merge pull request #13 from mc818/regular

y-axis changes