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

data being graphed:
![hist](https://cloud.githubusercontent.com/assets/529129/2726216/4a386182-c5b7-11e3-93ff-9b7a075037c3.png)

cmd
![cmd](https://cloud.githubusercontent.com/assets/529129/2726218/52dc3cdc-c5b7-11e3-93f9-9a1ac89c96bc.png)
before
![before](https://cloud.githubusercontent.com/assets/529129/2726220/57338704-c5b7-11e3-8be4-0c32d2e05a2d.png)
after
![after](https://cloud.githubusercontent.com/assets/529129/2726222/5b102f62-c5b7-11e3-8c27-b831491a2764.png)

cmd with -r
![cmd -r](https://cloud.githubusercontent.com/assets/529129/2726226/69c6962c-c5b7-11e3-85a5-d3af0284b9d8.png)
after with -r
![after -r](https://cloud.githubusercontent.com/assets/529129/2726227/69c79a86-c5b7-11e3-8776-6b18d7a2dc6d.png)